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.

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, Russian President Vladimir Putin turned blame for the fighting back on Ukraine, telling Turkish President Recep Tayyip Erdogan on Sunday that the invasion could be halted only “only if Kyiv ceases hostilities,” according to a Kremlin statement on the phone call.

He said Ukraine had to fulfill “the well-known demands of Russia,” which include what he calls the “demilitarization” and “denazification” of Ukraine.

The presidents of Turkey and France, 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, saying the Vatican would do everything it could to end the conflict.

“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.”

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.

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.

Separately, Ukraine’s national security service said Russian forces in the city of Kharkiv fired rockets at a physics institute that contains nuclear material and a reactor.

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 lead to a far wider war.

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

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.

As Russian forces surrounded several Ukrainian cities, Zelenskyy appeared on television Saturday night and rallied his people to stay defiant.

“Ukrainians in all of our cities that the enemy has entered — go on the offensive!” said Zelenskyy, who was wearing his habitual military green T-shirt. “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.

The death toll 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.

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 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.

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 health care workers in Ukraine, saying it verified at least six such attacks that have killed six people and wounded 11 others.

Attacks on health care 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.”