MUKACHEVO, Ukraine — Russian forces pounded key airfields in central Ukraine and launched a fresh assault on the besieged port city of Mariupol on Sunday, Russian and Ukrainian officials said, as Moscow pressed ahead with its invasion in defiance of new Western economic threats and fierce resistance from Ukraine’s outgunned defenders.

The newest attacks by Russian warplanes, missiles and artillery came as waves of refugees continued to pour across Ukraine’s northern border, and amid renewed pleadings from Ukrainian President Volodymyr Zelenskyy for international military help to “close the sky” to Russian bombers.

For the second consecutive day, Russian shelling ruptured a temporary cease-fire in Mariupol, blocking efforts to evacuate civilians in the Black Sea city where more than 200,000 residents remained trapped, according to a tally by relief agencies. In Irpin, on the outskirts of Kyiv, Ukraine’s capital, at least eight people, including two children, were killed in an artillery barrage as families were preparing to board buses to flee the area.

More than 1.5 million refugees from Ukraine have fled to neighboring countries over the past 10 days, the U.N. high commissioner for refugees, Filippo Grandi, said Sunday. He tweeted that the mass exodus is “the fastest growing refugee crisis in Europe since World War II.” Grandi recently predicted that more than 4 million people could be displaced by the conflict in the weeks to come.

Ukrainians braced for further violence as the invasion entered its 11th day. Zelenskyy warned of a coming Russian aerial assault on Odessa, the historic city of nearly 1 million people on the Black Sea coast, and Ukrainian officials reported steady advances by Russian armored columns in the country’s southeast. At the Zaporizhzhia nuclear power plant in southeastern Ukraine, an international nuclear watchdog accused Russian occupiers of interfering with the facility’s Ukrainian management. Russian troops seized Europe’s largest nuclear plant on Friday after a projectile set part of the complex on fire and heightened fears across Europe of a catastrophic accident.

The relentless attacks prompted new warnings from President Joe Biden’s administration and several NATO allies of harsher measures against Russia, from war-crimes investigations to possible restrictions against oil exports, which are an essential pillar of Russia’s economy.

Secretary of State Antony Blinken said the administration was in “very active discussions” with European partners on possibly blocking Russian oil sales, and Republican and Democratic lawmakers suggested that such a move would receive bipartisan support in Congress.

“What Vladimir Putin is doing is not only terrible violence to men, women, and children, he’s doing terrible violence to the very principles [that] keep peace and security around the world,” Blinken said during a visit on Sunday to Moldova, Ukraine’s southwestern neighbor that is now worried that it could be Putin’s next target. “We can’t let either of those things go forward with impunity, because if we do, it opens a Pandora’s box that we will deeply, deeply regret.”

Russian attacks on two key aviation facilities raised new concerns about the Ukraine’s ability to challenge Moscow’s control of the skies. Air strikes targeted Ukraine’s Starokostiantyniv military air base, about 150 miles southwest of Kyiv, as well as a commercial airport at Vinnystia, about 70 miles to the southeast. While the damage could not be independently assessed, the attacks could deprive Ukraine of usable airstrips as the country presses Western allies to send fighter planes to combat Moscow’s invasion.

A spokesman for the Russian Defense Ministry confirmed that the military had struck the air base with long-range, high precision weapons — apparently including cruise missiles, Among the targets was a Russian-made air defense system owned by Ukraine, the spokesman said.

“Almost all combat-capable aviation of the regime in Kyiv has been destroyed,” Russian Defense Ministry spokesman Igor Konashenkov said in a statement following the attacks.

Zelenskyy confirmed in a video message that the strike on Vinnystia had “completely destroyed the airport.”

In the same message, Zelenskyy, who has repeatedly urged NATO to help him defend his country against Russian warplanes, again called for assistance in fighting an air war.

“We repeat every day: Close the sky over Ukraine. Close it for all Russian rockets. For all Russian military aviation. For all these terrorists. Make a humanitarian airspace,” Zelenskyy said. “We are people, and this is your humanitarian obligation to protect us.”

Failing that, supply “airplanes so that we can protect ourselves,” he added.

For the second time in 24 hours, Russia was accused of violating cease-fire agreements intended to evacuate civilians from besieged cities. In Mariupol, the city council said evacuations were not possible because “Russians began to regroup their forces and to shell the city heavily.” A temporary truce to allow people to leave there and other places broke down less than 24 hours earlier.

In calls with French and Turkish leaders Sunday, Russian President Vladimir Putin vowed to press on with the invasion unless Ukraine stopped fighting.

It was time for Ukraine to “show a more constructive approach that fully takes into account the emerging realities,” he said, according to the Kremlin, in an apparent reference to Ukraine’s military and territorial losses since Russia’s invasion. Speaking by phone with Turkish President Recep Tayyip Erdogan, Putin said the war was “going according to plan” and on time. He denied that Russia was responsible for the civilian casualty toll, according to a Russian readout of the call.

Blinken said Sunday that the United States is exploring how it might supply Ukraine with fighter jets from NATO nations. “I can’t speak to the timeline but I can just tell you that we’re looking at it very, very actively,” Blinken said.

But Russia warned Sunday that foreign countries hosting Ukrainian combat aircraft could be viewed by Moscow as parties to the conflict.

“We know for a fact about Ukrainian combat planes which earlier flew to Romania and other neighboring countries,” Konashenkov said Sunday. “We would like to point out that the use of the network of airfields of those countries for the stationing of Ukrainian combat aviation for the further use against the Russian Armed Forces could be viewed as the involvement of those countries in the armed conflict,” he said.

In Irpin, outside Kyiv, video published Sunday showed a man wearing a yellow armband, usually worn by Ukrainian forces, and carrying a gun over his shoulder as he stands across from a church and sidewalk crowded with people carrying suitcases. He takes a few steps toward an intersection before an explosion rips through the middle of the street.

The area is covered in smoke. Someone runs out of the building and drags the man with the yellow armband out of the street. Soldiers sprint across the intersection to people collapsed on the ground, and someone shouts, “Medic!”

Associated Press photos of the aftermath show civilians — including children — killed in the attack. Lynsey Addario, a photographer working for the New York Times who witnessed the attack, said in a message posted on Twitter that “at least three members of a family of four were killed in front of me.”

Fahim reported from Istanbul, Warrick and Cahlan from Washington, and Ryan from Tallinn, Estonia. The Washington Post’s Jennifer Hassan in London and Danielle Paquette in Dakar, Senegal, contributed to this report.

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