LVIV, Ukraine--A second attempt to evacuate civilians from a besieged city in southern Ukraine collapsed Sunday during renewed Russian shelling, while Russian President Vladimir Putin shifted blame for the war to Ukraine and said Moscow's invasion could be halted “only if Kyiv ceases hostilities.”

Food, water, medicine and almost all other supplies were in desperately short supply in the port city of Mariupol, where Russian and Ukrainian forces had agreed to an 11-hour cease-fire that would allow civilians and the wounded to be evacuated. But Russian attacks quickly closed the humanitarian corridor, Ukrainian officials said.

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

The news dashed hopes that more people could escape the fighting in Ukraine, where Russia's plan to quickly overrun the country has been stymied by fierce resistance. Russia has made significant advances in southern Ukraine and along the coast, but many of its efforts have become stalled, including an immense military convoy that has been almost motionless for days north of Kyiv.

Ukrainian President Voldymyr Zelenskyy rallied his people to remain defiant, especially those in cities occupied by Russians.

“You should take to the streets! You should fight!” he said Saturday on Ukrainian television. “It is necessary to go out and drive this evil out of our cities, from our land.”

Zelenskyy also asked the United States and NATO countries to send more warplanes to Ukraine, though that idea is complicated by questions about which countries would provide the aircraft and how those countries would replace the planes.

He later urged the West to tighten its sanctions on Russia, saying that “the audacity of the aggressor is a clear signal" that existing sanctions are not enough.

The war, now in its 11th day, has caused 1.5 million people to flee the country. The head of the U.N. refugee agency called the exodus “the fastest-growing refugee crisis in Europe since World War II."

As he has often done, Putin blamed Ukraine for the war, telling Turkish President Recep Tayyip Erdogan on Sunday that Kyiv needed to stop all hostilities and fulfill “the well-known demands of Russia."

Putin launched his invasion with a string of false accusations against Kyiv, including that it is led by neo-Nazis intent on undermining Russia with the development of nuclear weapons.

The Russian Defense Ministry on Sunday announced that its forces intend to strike Ukraine’s military-industrial complex with what it said were precision weapons. A ministry spokesman, Igor Konashenkov, claimed in a statement carried by the state news agency Tass that Ukrainian personnel were being forced to repair damaged military equipment so that it could be sent back into action.

Zelenskyy criticized Western leaders for not responding to Russia's latest threat.

“I didn’t hear even a single world leader react to this,” Zelenskyy said Sunday evening. “The audacity of the aggressor is a clear signal to the West that the sanctions imposed on Russia are not sufficient.”

Putin and French President Emmanuel Macron spoke about the nuclear situation in Ukraine, which has 15 nuclear reactors at four power plants and was the scene of the 1986 Chernobyl nuclear disaster.

The men agreed in principle to a “dialogue” involving Russia, Ukraine and the U.N.'s atomic watchdog, according to a French official who spoke on condition of anonymity, in line with the presidency’s practices. Potential talks on the issue are to be organized in the coming days, he said.

Putin also blamed the fire last week at the Zaporizhzhia nuclear plant, which Ukrainian officials said was caused by Russian attackers, on a “provocation organized by Ukrainian radicals."

“Attempts to shift responsibility for this incident onto the Russian military are part of a cynical propaganda campaign,” he said, according to the French official.

Separately, Ukraine’s national security service says 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.

International leaders, as well as Pope Francis, appealed to Putin to negotiate.

In a highly unusual move, the pope said he had dispatched two cardinals to Ukraine to try to end the conflict.

“In Ukraine, rivers of blood and tears are flowing,” the pontiff said in his traditional Sunday blessing.

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.

About eight civilians were killed by Russian shelling in the town of Irpin, on the northwest outskirts of Kyiv, according to Mayor Oleksander Markyshin. The dead included a family.

Video footage showed a shell slamming into a city street, not far from a bridge used by people fleeing the fighting. A group of fighters could be seen trying to help the family.

The handful of residents who managed to flee Mariupol before the humanitarian corridor closed said the city of 430,000 had been devastated.

“We saw everything: houses burning, all the people sitting in basements,” said Yelena Zamay, who fled to one of the self-proclaimed republics in eastern Ukraine held by pro-Russian separatists. “No communication, no water, no gas, no light, no water. There was nothing.”

British military officials compared Russia’s tactics to those Moscow used in Chechnya and Syria, where surrounded cities were pulverized by airstrikes and artillery.

“This is likely to represent an effort to break Ukrainian morale,” the U.K. Ministry of Defense said.

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 lead to a far wider war.

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

The day before, Zelenskyy pleaded with American lawmakers in a video call to help get more warplanes to Ukraine.

U.S. officials say Washington is discussing ways to get the planes to Ukraine in a complex scenario that would include sending American-made F-16s to former Soviet bloc nations, particularly Poland, that are now members of NATO. Those countries would then send Ukraine their own Soviet-era MiGs, which Ukrainian pilots are trained to fly.

But because of production backlogs on the U.S. warplanes, the Eastern European nations would essentially have to give their MiGs to the Ukrainians and accept U.S. promises that they would get F-16s as soon as that was possible. Adding to the difficulties is the fact that the next shipment of F-16s is destined for Taiwan, and the U.S. Congress would be reluctant to delay those deliveries.

The Russian military has warned Ukraine’s neighbors against hosting its warplanes, saying that Moscow may consider those counties part of the conflict if Ukrainian aircraft fly combat missions from their territory.

The death toll remains lost in the fog of war. The U.N. says it has confirmed just a few hundred civilian deaths but also warned that the number is a vast undercount.

Oleksiy Arestovich, an adviser to Zelenskyy, said 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.

Ukraine’s military is greatly 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 resistance.

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 Azov. 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 alone.

Russia has become increasingly isolated in the days since the invasion began, closing itself off to outside sources of information as sanctions bite deeply into its economy. The ruble has plunged in value, and dozens of multinational companies ended or dramatically scaled back their work in the country.

On Sunday, American Express announced it would suspend operations in Russia, as well as in Russian-allied Belarus. A few hours later, TikTok said Russian users would not be able to post new videos or see videos shared from elsewhere in the world. The company blamed Moscow’s new “fake news” law, which makes it illegal, among other things, to describe the fighting as an invasion. Netflix also cut its service to Russia but provided no details.

Facebook and Twitter have already been blocked in Russia, along with access to the websites of a number of major international media outlets. TikTok is part of the Chinese tech company ByteDance.