Russia has asked China for military support and other aid in the time since it invaded Ukraine on Feb. 24, a senior U.S. official told ABC News.

China and Russia recently strengthened their partnership, and Chinese President Xi Jinping has stood by Russian President Vladimir Putin as he's bombarded Ukraine.

On Sunday, President Joe Biden’s top national security adviser, Jake Sullivan, said during an interview with CNN that the U.S. was "watching closely to see the extent to which China actually does provide any form of support, material support or economic support, to Russia."

"It is a concern of ours," Sullivan said, adding that the U.S. has communicated to Beijing that it will "not stand by and allow any country to compensate Russia for its losses from the economic sanctions."

Sullivan is planning to meet a top Chinese official in Rome on Monday.

The Financial Times, The Washington Post and The New York Times first reported on this development.

-ABC News' Ben Gittleson

The humanitarian corridor established for civilians fleeing the city of Mariupol was blocked again Sunday as Russian forces cast relentless airstrikes on the city, Ukrainian President Volodymyr Zelenskyy said in an address.

Zelenskyy continued to appeal for NATO to establish a no-fly zone in the region, saying NATO countries are next if it is not done soon.

“If you do not close our sky, it is only a matter of time before Russian missiles fall on your territory,” Zelenskyy warned NATO.

In western Ukraine, 30 missiles hit the region around Lviv, where the shelling of the International Center for Peacekeeping and Security killed 35 people and wounded 134 others, Zelenskyy said.

At least 10 humanitarian corridors operated in the Kyiv and Luhansk regions on Sunday, Zelenskyy said.

-ABC News' Christine Theodorou

Mykhailo Podolyak, an adviser to the office of Ukrainian President Volodymyr Zelenskyy and the lead negotiator in Ukraine-Russia talks, is urging world leaders to take a stance on the conflict Russia has sparked in Ukraine, saying the third world war is already among us.

"It’s already World War III –- world leaders must now choose their side," Podolyak told ABC News. "Every leader of every country must explain why he will stand to the side at the time when Russia was shelling civilian children in Ukraine."

Podolyak is "cautiously optimistic" that negotiations are progressing, he said, adding that Ukraine will not offer any concessions to Russia.

"My cautious optimism is based on the fact that they understand they will not break us and they will not have enough resources to answer what they started," he said.

Podolyak said that, despite what he described as "Russian propaganda" reports, no foreign fighters were killed during the strike on Yavoriv, on the outskirts of Lviv, on Sunday. He accused Russia of using the ceasefire -- which was implemented to establish humanitarian corridors -- to regroup and reposition themselves for strategic attacks.

In addition, the bombing and shelling of civilians in Southern Ukraine continues to intensify, Podolyak said.

More negotiations between Ukraine and Russia are schedule to take place tomorrow, Podolyak said.

-ABC News' Maggie Rulli

The United Nations High Commissioner for Refugees has calculated that 2,698,280 people have fled Ukraine since Feb. 24 as a result of the Russian invasion.

About 1.66 million people have entered Poland, 246,206 people have entered Hungary, and 195,980 people have entered Slovakia.

Nearly 200,000 people have entered Russia, Moldova, Romania and Belarus combined, while more than 304,000 people have entered other European countries.

-ABC News' Zoe Magee