Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

Secretary of State Antony Blinken warned Wednesday that he thinks the atrocities in the port city of Mariupol will show to be "far worse" than in Bucha, where, after Russian troops retreated, there were found to be scores of civilians killed, and in some cases, tortured.

"We can only anticipate that when this tide also at some point recedes from Mariupol, we're going to see far worse -- if that's possible to imagine," Blinken said during a press conference in Panama.

Blinken expressed some doubts about Russia's offer of humanitarian corridors for Ukrainian civilians and service members to leave Mariupol.

"Of course, we want to see people who are in harm's way, if they're able to, leave it safely and securely," he said. "The judgment on whether the humanitarian corridor established to do that from Mariupol is safe and secure is one ultimately that the Ukrainian government is going to make … and ultimately, the decision to leave is going to be a burden on the people themselves to make that very difficult decision."

He said the U.S. is "certainly assisting" by giving its evaluations and assessments of the situation to the Ukrainian government.

But Blinken warned: "What gives pause is the fact that there have been agreements on humanitarian corridors established before that have fallen apart very, very quickly - if not immediately - principally because the security has been violated by Russian forces."

-ABC News' Conor Finnegan

The Biden administration is preparing to announce another military aid package for Ukraine worth roughly $800 million, a U.S. official said. Details have not been determined but the package is believed to focus more on artillery and ammunition.

-ABC News' Conor Finnegan

Treasury Secretary Janet Yellen will hold a bilateral meeting with Ukrainian Prime Minister Denys Shmyhal in Washington, D.C., on Thursday, a Treasury Department spokesperson confirmed. Ukraine's finance minister will also attend.

The meeting will take place during the annual conferences of the International Monetary Fund and the World Bank. The conferences draw leaders from the G20, of which Russia is currently a member.

While Yellen will participate in some meetings and panels where Russian officials will be present, she will skip others, a Treasury Department official said.

-ABC News' Sarah Kolinovsky

Tennis players from Russia and Belarus have been banned from this summer's Wimbledon Championships due to the "unjustified and unprecedented military aggression" in Ukraine, The All England Lawn Tennis Club said.

"It would be unacceptable for the Russian regime to derive any benefits from the involvement of Russian or Belarusian players," the statement said, adding: "If circumstances change materially between now and June, we will consider and respond accordingly."