LVIV, Ukraine — Plans to evacuate civilians from a besieged port city in Ukraine collapsed Sunday for the second time along with an expected Russian cease-fire, Ukrainian officials said as they tried to persuade Russia to agree on terms for safely getting residents out of areas under fire near Ukraine’s capital.

Residents expected to leave the port city of Mariupol during a 10 a.m. to 9 p.m. local ceasefire, Ukrainian military authorities said earlier in the day. Interior Ministry adviser Anton Gerashchenko said the planned evacuations were halted because of an ongoing assault by Russian troops.

“There can be no ‘green corridors’ because only the sick brain of the Russians decides when to start shooting and at whom,” Gerashchenko said on Telegram.

The news dashed hopes of progress in easing, much less ending, the war in Ukraine, which is now in its 11 day and has caused 1.5 million people to flee the country. The head of the U.N. refugee agency on Sunday called the exodus “the fastest-growing refugee crisis in Europe since World War II.

The presidents of Turkey and France, as well as Pope Francis, appealed to Russian President Vladimir Putin to negotiate to end the conflict.

Separately, Ukraine’s national security service said Russian forces fired rockets at a physics institute in the city of Kharkiv that contains nuclear material and a reactor. Russian troops already took control of the Zaporizhzhia plant in Ukraine, as well as Chernobyl, the site of the world’s worst nuclear disaster.

The security service said a strike on the nuclear facility in Kharkiv could lead to “large-scale ecological disaster.” The service said on Facebook Sunday that the Russians were firing from Grad launchers. Those missiles do not have precise targeting, raising concern that one would go astray.

Ukrainian President Voldymyr Zelenskyy reiterated a request for foreign protectors to impose a no-fly zone over Ukraine, which NATO so far has ruled out because of concerns such an action would draw the West into the war.

“The world is strong enough to close our skies,” Zelenskyy said in a video address on Sunday.

Putin warned Saturday that Moscow would consider a third-party declaration to close Ukrainian airspace to be a hostile act.

The disappointment for women, children and older adults who waited to leave Mariupol and the nearby city of Volnovakha while able Ukrainian men stayed behind to fight came after a similar cease-fire deal collapsed Saturday and foreign leaders sought to bring diplomacy to bear on ending the war.

Putin told Turkish President Recep Tayyip Erdogan that Russia’s invasion of Ukraine could be halted only “only if Kyiv ceases hostilities and fulfills the well-known demands of Russia,” according to the Kremlin’s readout of the phone call the two leaders held on Sunday.

Putin earlier listed “demilitarization” and “denazification” of Ukraine, recognition of Crimea as part of Russia, and recognition of the separatist regions in eastern Ukraine as independent states as the Kremlin’s main demands.

“Hope was expressed that during the planned next round of negotiations, the representatives of Ukraine would show a more constructive approach, fully taking into account the emerging realities.” The third round of talks between Russian and Ukrainian negotiators is scheduled for Monday.

In a highly unusual move, Pope Francis said Sunday that he had dispatched two cardinals to Ukraine, saying the Vatican was will to do everything it could to bring peace to end a conflict that began on Feb. 24 when Russia invaded Ukraine.

“In Ukraine, rivers of blood and tears are flowing,” the pontiff said in his traditional Sunday blessing. “This is not just a military operation, but a war that sows death, destruction and misery.”

Oleksiy Arestovich, an adviser to Zelenskyy, said Sunday that Ukrainian officials and international humanitarian organizations were working with Russia through intermediaries to establish humanitarian corridors from Bucha and Hostomel, which are Kyiv suburbs where there has been heavy fighting.

After the cease-fire in Mariupol failed to hold Saturday, Russian forces intensified their shelling of the city and dropped massive bombs on residential areas of Chernihiv, a city north of Kyiv, Ukrainian officials said.

In Mariupol, bereft mothers mourned slain children, wounded soldiers were fitted with tourniquets and doctors worked by the light of their cellphones as bleakness and desperation pervaded.

On Saturday, Putin pinned the blame for the war on the Ukrainian leadership, slamming their resistance to Russian forces.

“If they continue to do what they are doing, they are calling into question the future of Ukrainian statehood,” the Russian leader said. “And if this happens, it will be entirely on their conscience.”

He also hit out at Western sanctions that have crippled Russia’s economy and sent the value of its currency tumbling. Meanwhile, more companies are suspending operations in Russia, including Mastercard and Visa.

“These sanctions that are being imposed, they are akin to declaring war,” he said.

As Russian forces surrounded several Ukrainian cities and maintained a convoy outside the capital, Kyiv, Zelenskyy appeared on television Saturday night wearing what has become a habitual military green T-shirt and rallied his people to remain defiant.

“Ukrainians in all of our cities that the enemy has entered — go on the offensive!” Zelenskyy said. “You should take to the streets! You should fight! … It is necessary to go out and drive this evil out of our cities, from our land.”

The strength of Ukrainian resistance continues to surprise Russian forces, and they have responded by targeting populated areas, including the cities of Kharkiv, Chernihiv and Mariupol, the U.K. Ministry of Defense said in its daily intelligence briefing.

British military officials on Sunday compared Russia’s tactics in Ukraine to those used in Chechnya and Syria, where surrounded cities were pummeled with airstrikes and artillery after Russian forces faced unexpected resistance.

“This is likely to represent an effort to break Ukrainian morale,” the ministry said. “Russia has used similar tactics in Chechnya in 1999 and Syria in 2016, employing both air and ground-based munitions.”

The death toll of the conflict is difficult to measure. The U.N. human rights office said at least 351 civilians have been confirmed killed, but the true number is probably much higher. Russian and Ukrainian defense officials have not provided information on their military causalities.

Ukraine’s military is vastly outmatched by Russia’s, but its professional and volunteer forces have fought back with fierce tenacity. In Kyiv, volunteers lined up Saturday to join the military.

Even in cities that have fallen, there were signs of the resistance Zelenskyy requested.

Onlookers in Chernihiv cheered as they watched a Russian military plane fall from the sky and crash, according to video released by the Ukrainian government. In Kherson, hundreds of protesters waved blue and yellow Ukrainian flags and shouted, “Go home.”

Russia has made significant advances in southern Ukraine as it seeks to block access to the Sea of Avrov. Capturing Mariupol could allow Moscow to establish a land corridor to Crimea, which Russia annexed from Ukraine in 2014 in a move that most other countries considered illegal.

The West has broadly backed Ukraine, offering aid and weapon shipments and slapping Russia with vast sanctions. But no NATO troops have been sent to Ukraine, leaving Ukrainians to fight Russian troops.

U.S. President Joe Biden called Zelenskyy early Sunday, Kyiv time, to discuss Russia sanctions and speeding U.S. assistance to Ukraine. The White House said the conversation also covered talks between Russia and Ukraine but did not give details.

U.S. Secretary of State Antony Blinken spent the weekend visiting NATO member nations in Eastern Europe that have taken in refugees from Ukraine. In Moldova on Sunday, he pledged support for the western-leaning former Soviet republic that is warily watching Russia’s moves in Ukraine.

The World Health Organization on Sunday condemned attacks on healthcare workers in Ukraine, saying it verified at least six such attacks that have killed six people and injured 11 others.

Attacks on healthcare workers are a violation of international humanitarian law, WHO Director-General Tedros Adhanom Ghebreyesus said on Twitter.

The U.N. said it would increase its humanitarian operations both inside and outside Ukraine, and the Security Council scheduled a meeting for Monday on the worsening situation.

The U.N. World Food Program has warned of an impending hunger crisis in Ukraine, a major global wheat supplier, saying millions will need food aid “immediately.”