WASHINGTON — In an interview Thursday with NBC News, US President Joe Biden repeated his warning that any Americans still in Ukraine should leave as soon as possible.

“It’s not like we’re dealing with a terrorist organization. We’re dealing with one of the largest armies in the world. It’s a very different situation, and things could go crazy quickly,” he said.

Asked whether there were any scenarios that would prompt him to send U.S. troops to Ukraine to rescue Americans, the president said: “There’s not. That’s a world war when Americans and Russia start shooting at one another.”

Μeanwhile, Britain’s defense secretary visited Moscow on Friday in another effort to ease tensions over a possible invasion of Ukraine by Russia, which held massive war games near its neighbor and insisted the tensions were “not at our fault.”

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Ben Wallace’s trip came a day after British Foreign Secretary Liz Truss also held talks in Moscow, urging Russia to pull back over 100,000 troops near Ukraine and warning that attacking its neighbor would “have massive consequences and carry severe costs.”

Russia says it has no plans to invade but wants the West to keep Ukraine and other former Soviet countries out of NATO. It also wants NATO to refrain from deploying weapons there and to roll back alliance forces from Eastern Europe — demands flatly rejected by the West.

Speaking at the start of his talks with Wallace, Russian Defense Minister Sergei Shoigu noted that “the military-political situation in Europe is growing increasingly tense, and not at our fault.”

Amid the soaring tensions, British Prime Minister Boris Johnson warned Thursday that the Ukraine crisis has grown into “the most dangerous moment” for Europe in decades.

Russia’s troop concentration includes forces deployed on the territory of its ally Belarus for massive joint drills involving firing live ammunition. Those exercises entered a decisive phase Thursday and will run through Feb. 20. The Ukrainian capital is about 75 kilometers (47 miles) south of the Belarus border.

Continuing its military buildup near Ukraine, Russia has moved six amphibious assault vessels into the Black Sea, augmenting its capability to land marines on the coast.

Moscow has announced sweeping drills in the Black and Azov seas in the coming days and closed large areas for commercial shipping, drawing a strong protest from Ukraine on Thursday.

NATO has stepped up military deployments to bolster its eastern flank, with the U.S. sending troops to Poland and Romania.

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The U.S. Navy said Thursday that it has deployed four destroyers from the United States to European waters. The Navy did not directly tie this deployment to the Ukraine crisis but said the ships provide “additional flexibility” to the U.S. Sixth Fleet commander, whose area of responsibility includes the Mediterranean, and will operate in support of NATO allies.

NATO Secretary-General Jens Stoltenberg visited a military base in Romania, hailing the ongoing deployment of 1,000 additional U.S. troops that will nearly double their current number there.

“This is a powerful demonstration of trans-Atlantic unity,” Stoltenberg said.

Russian Foreign Ministry spokeswoman Maria Zakharova responded by noting that “NATO keeps building up its presence near Russia’s borders and exacerbates the situation around Ukraine to create a pretext for that.”

Russia and Ukraine have been locked in a bitter conflict since 2014, when Ukraine’s Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing Crimea and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people.

A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled. The Kremlin has accused Kyiv of sabotaging the agreement, and Ukrainian officials argued in recent weeks that implementing it would hurt their country.

Foreign policy advisers from Germany, France, Russia and Ukraine held nearly nine hours of talks in Berlin on Thursday to try to revive the stalled agreement but made no progress.

Russian representative Dmitry Kozak said Ukraine firmly refused to commit to a dialogue with the rebels on a political settlement, blocking any further movement. Ukrainian envoy Andriy Yermak sounded a more positive note, noting that the parties agreed to continue their discussions and hailed the four-way talks as an “effective and efficient platform.”

The Berlin talks were part of renewed diplomatic efforts to resolve the biggest security crisis between Russia and the West since the Cold War.

German Chancellor Olaf Scholz, who discussed the standoff with Biden earlier this week and plans to visit Kyiv and Moscow on Feb. 14 and Feb. 15, renewed his warning to Russia.

“It is our job to ensure that we prevent a war in Europe, in that we send a clear message to Russia that any military aggression would have consequences that would be very high for Russia and its prospects, and that we are united with our allies,” Scholz told the German parliament’s upper house on Friday.

Pointing to his trips next week, the chancellor said “everything serves to achieve this: securing peace in Europe, and that is worth every effort.”