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

Britain's Ministry of Defense is claiming that Russian missile strikes across Ukraine are an attempt to hamper Ukrainian resupply efforts.

As Russian forces struggled, they targeted civilians, including at homes, transit hubs, schools and hospitals, "in an attempt to weaken Ukrainian resolve," the Ministry of Defense's intelligence update said.

The U.K. believes Russia's focus on Odesa, Kherson and Mariupol reflect its "desire to fully control access to the Black Sea, which would enable them to control Ukraine’s sea lines of communication, negatively impacting their economy," the intelligence update said.

-ABC News' Christine Theodorou

Russian troops have entered part of the Azovstal Iron and Steel Works plant in Mariupol, Ukraine's chief negotiator with Russia, David Arakhamia, said in an interview with Ukraine's Radio Liberty on Wednesday.

The plant continues to come under bombardment and shelling, he said.

The plant, which stretches over 4.2 square miles, is the last pocket of Ukrainian resistance in Mariupol. Russia claimed Wednesday that its military had taken complete control of Mauripol, a strategic port city in Ukraine's war-torn east.

This is the first time it appears that Russian soldiers have successfully entered the plant. It is not clear how many soldiers entered or where.

-ABC News' Fidel Pavlenko

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn