KYIV, Ukraine — US Secretary of State Antony Blinken urged western nations on Wednesday to remain united in the face of what he called “relentless” Russian aggression against Ukraine and reassured Ukraine’s leader of their support while calling for Ukrainians to stand strong.

Blinken told Ukrainian President Volodymyr Zelenskyy during a visit to Kyiv that the U.S. and its allies were steadfast in backing his country and its democratic aspirations amid growing fears of a potentially imminent Russian invasion.

Blinken said Russia had plans to boost its military presence of some 100,000 troops along the Ukrainian border and suggested that number could double soon. Blinken also said he would not be presenting a formal written response to Russia’s demands when he meets on Friday with Russia’s foreign minister.

“The Ukrainian people chose a democratic and European path in 1991. They took to the Maidan to defend that choice in 2013, and unfortunately ever since you have faced relentless aggression from Moscow,” Blinken said, referring to Ukraine’s trajectory since the collapse of the Soviet Union.

“Our strength depends on preserving our unity and that includes unity within Ukraine,” he told Zelenskyy. “I think one of Moscow’s long-standing goals has been to try to sow divisions between and within our countries and quite simply, we cannot and will not let them do that.”

The Biden administration said earlier it was providing an additional $200 million in defensive military aid to Ukraine to help protect its sovereignty and territorial integrity.

Zelenskyy thanked Blinken for the assistance, which was approved in late December but not formally confirmed until Wednesday, as well as for his visit and assurances of support.

“This (military) support not only speaks to our strategic plans of Ukraine joining the alliance, but more importantly to the level of our military, our military supplies,” he said, referring to Kyiv’s desire to join NATO over Russia’s strong objections.

“If we want dramatically fast steps in modernizing the military, we need help especially in these tough times,” Zelenskyy said. “Your visit is very important. It underlines once again your powerful support of our independence and sovereignty.”

The aid announcement came at the start of Blinken’s hastily arranged visit as U.S. and western officials stepped up increasingly dire warnings about a possible Russian invasion.

Blinken said Russian President Vladimir Putin is now in position to launch military action against Ukraine at will and at very short notice with more than 100,000 troops massed on its border and plans to add more. He said that number “could double on relatively short order,” but he did not provide details.

“We know that there are plans in place to increase that force even more on very short notice and that gives President Putin the capacity, also on very short notice, to take further aggressive action against Ukraine,” Blinken told staff at the U.S. Embassy in Kyiv.

After his meetings with Zelenskyy and other senior Ukrainian officials, Blinken plans a short trip to Berlin for talks with German and other European allies on Thursday, He is scheduled to see his Russian counterpart, Sergey Lavrov, in Geneva on Friday. That meeting is aimed at testing Russia’s willingness to resolve the crisis diplomatically, officials said.

The administration and its European allies have accused Putin of creating the crisis by massing troops along Ukraine’s borders and it is up to him and the Russians to decide whether to invade and suffer severe economic consequences.

Russia has brushed off calls to withdraw its troops by saying it has a right to deploy its forces wherever it likes on its own territory. It also has rejected U.S. allegations that it’s preparing a pretext to invade Ukraine. Lavrov dismissed the U.S. claim as “total disinformation.”

The U.S. has not concluded whether Putin plans to invade or whether the show of force is intended to squeeze security concessions without an actual conflict. Inconclusive diplomatic talks between Moscow and the West in Europe last week failed to resolve stark disagreements over Ukraine and other security matters.

Instead, those meetings appear to have increased fears of a Russian invasion, and the Biden administration has accused Russia of preparing a “false flag operation” to use as a pretext for intervention. Russia has angrily denied the charge.

CIA Director William Burns visited Kyiv last week to consult with his Ukrainian counterparts and discuss current assessments of the risk to Ukraine, a U.S. official said, speaking on condition of anonymity to discuss Burns’ schedule, which is classified.

Ahead of his face-to-face meeting with L avrov, Blinken spoke to the Russian foreign minister by phone on Tuesday and “stressed the importance of continuing a diplomatic path to de-escalate tensions,” the State Department said.

Lavrov reaffirmed that Russia expects a written response this week from the U.S. and its allies to Moscow’s request for binding guarantees that NATO will not embrace Ukraine or any other ex-Soviet countries or station its forces and weapons there.

Blinken underscored to Lavrov on Tuesday that any discussion of European security “must include NATO Allies and European partners, including Ukraine,” the State Department said.

The Russian Foreign Ministry said Lavrov emphasized in the call with Blinken the key aspects of Russian draft documents envisaging “legally binding guarantees of Russia’s security in line with the principle of indivisibility of security approved by all countries in the Euro-Atlantic.” It said Lavrov stressed the importance for Washington to quickly deliver a written response to the Russian proposals.

Washington and its allies firmly rejected Moscow’s demands during last week’s Russia-U.S. negotiations in Geneva and a related NATO-Russia meeting in Brussels and it does not appear likely the Biden administration will reply to Russia in written form.

Meanwhile, the White House is accusing Russia of deploying operatives to rebel-controlled eastern Ukraine to carry out acts of sabotage there and blame them on Ukraine to create a pretext for possible invasion.

Ahead of Blinken’s visit to Kyiv, a delegation of U.S. senators was visiting Ukraine to emphasize congressional support for the country.

Russia in 2014 seized the Crimean Peninsula after the ouster of Ukraine’s Moscow-friendly leader and also threw its weight behind a separatist insurgency in eastern Ukraine. More than 14,000 people have been killed in nearly eight years of fighting between the Russia-backed rebels and Ukrainian forces in the country’s industrial heartland called Donbas.

Putin has warned that Moscow will take unspecified “military-technical measures”

In comments to U.S. Embassy staff in Kyiv, Blinken went further by saying that Russian President Vladimir Putin has plans to significantly enhance Moscow’s military presence near Ukraine’s border, which now numbers roughly 100,000 troops.

“We know that there are plans in place to increase that force even more on very short notice and that gives President Putin the capacity, also on very short notice, to take further aggressive action against Ukraine,” Blinken said.

After his meetings with President Volodymyr Zelenskyy and other senior Ukrainian officials and a short trip to Berlin for talks with German and other European allies on Thursday, Blinken will see his Russian counterpart in Geneva on Friday. That meeting is aimed at testing Russia’s willingness to resolve the crisis diplomatically, officials said.

“We’re now at a stage where Russia could at any point launch an attack in Ukraine,” White House press secretary Jen Psaki said Tuesday. “And what Secretary Blinken is going to go do is highlight very clearly there is a diplomatic path forward.”

The administration and its European allies have accused Putin of creating the crisis by massing troops along Ukraine’s borders and it is up to him and the Russians to decide whether to invade and suffer severe economic consequences.

The U.S. has not concluded whether Putin plans to invade or whether the show of force is intended to squeeze security concessions without an actual conflict. Russia has brushed off calls to withdraw its troops by saying it has a right to deploy its forces wherever it likes on its own territory.

Blinken’s meetings follow inconclusive diplomatic talks between Moscow and the West in Europe last week that failed to resolve stark disagreements over Ukraine and other security matters.

Instead, those meetings appear to have increased fears of a Russian invasion, and the Biden administration has accused Russia of preparing a “false flag operation” to use as a pretext for intervention. Russia has angrily denied the charge.

From Kyiv, Blinken will travel to Berlin, where he will meet with his German, British and French counterparts to discuss a possible response to any Russian military action. In Geneva on Friday, Blinken will be testing Russian Foreign Minister Sergey Lavrov on Russia’s interest in a “diplomatic off-ramp” for the crisis, the State Department said.

“The trip follows extensive diplomacy with our European Allies and partners about a united approach to address the threat Russia poses to Ukraine and our joint efforts to encourage it to choose diplomacy and de-escalation in the interests of security and stability,” the department said.

CIA Director William Burns visited Kyiv last Wednesday to consult with his Ukrainian counterparts and discuss current assessments of the risk to Ukraine, a U.S. official said, speaking on condition of anonymity to discuss Burns’ schedule, which is classified. While there, he also discussed the current situation with Zelenskyy and efforts to de-escalate tensions.

Blinken spoke by phone Tuesday with Lavrov, discussing the diplomatic talks and meetings held last week. The State Department said Blinken “stressed the importance of continuing a diplomatic path to de-escalate tensions” surrounding the Russia-Ukraine situation and “reiterated the unshakable U.S. commitment” to Ukraine’s sovereignty and territorial integrity.

Russia has rejected the U.S. allegations that it’s preparing a pretext to invade Ukraine. Lavrov dismissed the U.S. claim as “total disinformation.”

Lavrov reaffirmed that Russia expects a written response this week from the U.S. and its allies to Moscow’s request for binding guarantees that NATO will not embrace Ukraine or any other ex-Soviet countries or station its forces and weapons there.

Blinken underscored to Lavrov on Tuesday that any discussion of European security “must include NATO Allies and European partners, including Ukraine,” the State Department said.

The Russian Foreign Ministry said Lavrov emphasized in the call with Blinken the key aspects of Russian draft documents envisaging “legally binding guarantees of Russia’s security in line with the principle of indivisibility of security approved by all countries in the Euro-Atlantic.” It said Lavrov stressed the importance for Washington to quickly deliver a written response to the Russian proposals.

Washington and its allies firmly rejected Moscow’s demands during last week’s Russia-U.S. negotiations in Geneva and a related NATO-Russia meeting in Brussels.

The White House said Friday that U.S. intelligence officials had concluded that Russia had already deployed operatives to rebel-controlled eastern Ukraine to carry out acts of sabotage there and blame them on Ukraine to create a pretext for possible invasion.

Ahead of Blinken’s visit to Kyiv, a delegation of U.S. senators was visiting Ukraine to emphasize congressional support for the country.

Speaking Monday on a visit to Kyiv, German Foreign Minister Annalena Baerbock warned that “any further escalation would carry a high price for the Russian regime — economic, political and strategic,” and she emphasized the need to continue negotiations.

Russia in 2014 seized the Crimean Peninsula after the ouster of Ukraine’s Moscow-friendly leader and also threw its weight behind a separatist insurgency in eastern Ukraine. More than 14,000 people have been killed in nearly eight years of fighting between the Russia-backed rebels and Ukrainian forces in the country’s industrial heartland called Donbas.

Putin has warned that Moscow will take unspecified “military-technical measures” if the West stonewalls its demands.