Russia-Ukraine updates: US sanctions Russian military shipbuilder, diamond miner

A fourth round of talks between Russia and Ukraine are due to begin on Monday, following optimistic comments from both sides over the weekend that they are moving towards a compromise.

Both sides have confirmed the latest round of the talks will take place today -- the previous three rounds were held in Belarus, but these will take place remotely.

On Sunday, one of Russia’s negotiators, an MP Leonid Slutsky told Russian media that he believed “substantial progress” had been made and that he believed that progress could even “grow into a unified position” in documents for signing in the next few days.

Ukraine’s chief negotiator, Mykhailo Podolyak, in interviews and videos posted on Twitter on Sunday also said that Russia “looks at the situation far more properly” and has stopped throwing out “ultimatums.”

Podolyak told the Russian newspaper Kommersant the sides were discussing concrete proposals and that the key issue was "security guarantees” for both Russia and Ukraine. He said the sides were discussing a cease-fire, as well as compensation to Ukraine’s infrastructure destroyed during the war. But he did say that “some time is still needed” for Russia to understand the reality of its situation.

The comments have raised hopes Russia may be lowering its war aims as a result of the fierce Ukrainian resistance and tough response from Western countries.

U.S. Deputy Secretary of State Wendy Sherman told Fox News on Sunday that the U.S. also sees Russia is showing signs of a "willingness to have real, serious negotiations.”

But is unclear where the compromise might be found.

Last week, Russia was insisting that Ukraine change is constitution to guarantee it will not join NATO or the European Union. Ukraine had signalled that was not possible but President Volodymyr Zelenskyy has hinted there might be some negotiating space around NATO, which he has acknowledged Ukraine is not close to joining.

In a video posted to Twitter Monday morning before the start of the talks, Podolyak said Ukraine’s positions were “unchanged”: it was demanding an immediate ceasefire and a withdrawal of Russian troops. He said only after that could any political settlements be discussed.

-ABC News' Patrick Reevell

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