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

Ukrainian President Volodymyr Zelenskyy told reporters that U.S. Secretary of State Antony Blinken and Defense Secretary Lloyd Austin will visit Ukraine on Sunday.

Zelenskyy said they will discuss the list of weapons Ukraine needs and their delivery date.

A spokesperson for the State Department declined to comment and a spokesperson for the Defense Department said they had "nothing to offer" on the trip.

-ABC News' Jason Volack

British Prime Minister Boris Johnson told Ukrainian President Volodymyr Zelenskyy that Russia would be held to account for its actions and that the British government is helping collect evidence of war crimes, when the two spoke on Saturday.

Johnson confirmed that the U.K. will provide Ukraine with more defensive military aid including protected mobility vehicles, drones and anti-tank weapons.

He also updated Zelenskyy on new U.K. sanctions against members of the Russian military and confirmed the U.K. would reopen its embassy in Kyiv next week.

Ahead of the U.N. Secretary-General’s meetings with Zelenskyy and Russian President Vladimir Putin, both leaders agreed on the importance of establishing a cease-fire and humanitarian corridor to allow civilians to leave Mariupol, according to a statement from the British government.

The leaders discussed the U.K.'s efforts with partners to reach a long-term security solution for Ukraine, including discussions with international partners to provide more financial support to Ukraine.

Russian forces attacked a steel plant in the shattered Ukrainian port city of Mariupol on Saturday, according to Oleksiy Arestovich, an adviser to the Ukrainian president's office.

In a briefing, Arestovich said Russian forces had resumed airstrikes on the plant and were trying to storm it, a reversal of Russian President Vladimir Putin's order two days earlier.

Ukrainian officials estimate that about 1,000 civilians are sheltered in the plant along with the remaining Ukrainian fighters.

Busses promised by Russia to evacuate people from Mariupol did not arrive on Saturday.

At least 200 Mariupol residents had gathered near the Port City shopping center to be evacuated to Zaporizhia, adviser to the mayor of Mariupol Petro Andryushchenko said on Facebook.

“Instead of the buses promised by the Russian side, the Russian military approached Mariupol residents and ordered them to go away because "there will be shelling now." In fact, with rough coercion, they dispersed people. The buses were not submitted for loading,” he wrote.