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

More than 300 civilians have been evacuated from the besieged Ukrainian port city of Mariupol and surrounding areas, officials said late Wednesday.

The International Committee of the Red Cross said it facilitated the safe passage of the civilians in coordination with the United Nations and both sides of the Russia-Ukraine conflict. The evacuees arrived Wednesday in Zaporizhzhia, a Ukrainian government-controlled city about 140 miles northwest of Mariupol.

"We are relieved that more lives have been spared," Pascal Hundt, the ICRC's head of delegation in Ukraine, said in a statement Wednesday night. "We welcome the renewed efforts of the parties with regards to safe passage operations. They remain crucial and urgent in light of the immense suffering of the civilians."

Ukrainian Deputy Prime Minister Iryna Vereshchuk confirmed that 344 people were evacuated to Zaporizhzhia from the Mariupol area, Manhush, Berdyansk, Tokmak and Vasylivka.

The evacuation did not include civilians trapped inside the Azovstal Iron and Steel Works plant, the last pocket of Ukrainian resistance in Mariupol.

-ABC News' Natalya Kushnir and Cindy Smith

Ukrainian military officer Denys Procopenko said Russians have breached the Azovstal Iron and Steel Works plant in Mariupol, where hundreds of civilians remain.

He said heavy fighting is ongoing.

Procopenko is commander of the Azov regiment, which is a far-right paramilitary that's now incorporated into Ukrainian government security forces.

The Russian Defense Ministry said a humanitarian corridor will open this week for the evacuation of civilians from the Azovstal Iron and Steel Works plant in Mariupol.

The humanitarian corridor will be open Thursday, Friday and Saturday from 8 a.m. to 6 p.m. Moscow time.

Russia said its forces will "cease any hostilities" during that time.

Ukrainian President Volodymyr Zelenskyy spoke with United Nations Secretary-General António Guterres on Wednesday, asking for the U.N.'s help in evacuating "all the wounded" from the plant.

"The lives of the people who remain there are in danger. Everyone is important to us," Zelenskyy said, according to a statement from his office.

Hundreds of civilians are believed to be trapped in the plant.

The plant, which stretches over 4.2 square miles, has been facing bombardment and shelling. It's the last pocket of Ukrainian resistance in Mariupol; Russia claimed Wednesday that its military had taken complete control of the city.

It appears Ukrainians have managed to push Russian forces back from Kharkiv, about 20 to 30 miles east of the city, a senior U.S. defense official said Wednesday.

"We still think though that the Russians want Kharkiv," the official added.

The Wagner Group -- a private military force linked to Russia -- has been operating in the Donbas region in eastern Ukraine, using fighters recruited from places including Syria and Libya, the official said.

But overall, Russia's momentum is slow, the official said.

"We haven't seen much progress by the Russians coming north out of Mariupol at all. They seem to have paused either to create better defensive positions or to refit and re-posture themselves," the official said.

"Most of the strikes continue to be focused on the JFO [Joint Forces Operation] and on Mariupol," the official said.

"We have seen some missile strikes out into the west near Lviv. Looks like they're trying to hit critical infrastructure -- electricity and that kind of thing, and trying to get at the ability for the Ukrainians to use railroads in particular," the official said, adding that there are no indications the Russians have successfully disrupted Ukrainian resupply efforts.

-ABC News' Matt Seyler