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

The Russian Ministry of Defense claimed Tuesday that a temporary cease-fire has been declared in the besieged city of Mariupol and that a humanitarian corridor has been opened to allow Ukrainian fighters to lay down their arms escape with their lives.

But Russian officials earlier stated that the Ukrainian fighters taking a stand at the Azovstal Iron and Steel Works plant in Mariupol had until 4 p.m. local time to surrender, a deadline that has now passed.

"For this purpose, a 'complete silence regime' has been introduced, any hostilities have been stopped, units of the Russian Armed Forces and the formations of the Donetsk People's Republic along the entire perimeter of Azovstal have been withdrawn to a safe distance," said Mikhail Mizintsev, head of the Russian National Defense Control Center.

Mizintsev said that the humanitarian corridor was opened "in view of the catastrophic situation at the Azovstal metallurgical plant in the city of Mariupol, and also guided by purely humane principles."

Ukrainian officials have not confirmed the opening of the humanitarian corridor out of Mariupol. Ukrainian Deputy Prime Minister Iryna Vereshchuk said earlier that humanitarian corridors would not be reopened on Tuesday.

Mizintsev said three humanitarian convoys have been set up in the immediate vicinity of the plant to take evacuees in three directions.

"Each humanitarian convoy includes 30 buses and vehicles for transporting people, 10 ambulances with medical and nursing teams," Mizintsev said. "In addition, meeting points and temporary accommodation have been deployed in all three directions, food points and primary medical care have been organized."

Russia is starting the next phase of its "special military operation" in neighboring Ukraine, according to Russian Foreign Minister Sergey Lavrov.

"This operation will continue. Another stage of this operation is beginning," Lavrov said in an interview Tuesday with English-language Indian television network India Today. "I am sure this will be a very important moment of this entire special operation."

Lavrov noted that the goal of the operation is to "fully liberate" the Russian-backed separatist regions of eastern Ukraine's Donetsk and Luhansk oblasts, "as it was declared from the very start."

Russia-backed separatist forces are trying to storm a steel plant in besieged Mariupol where Ukrainian troops are holed up, according to separatist spokesperson Eduard Basurin.

Basurin, spokesman of the militia for a pro-Russia breakaway republic of Donetsk in eastern Ukraine's disputed Donbas region, told Russian state media on Tuesday that a separatist special forces unit was chosen to assist the Russian military in storming the giant plant of the Azovstal Iron and Steel Works company in Mariupol, a strategic port city in eastern Ukraine's war-torn Donetsk Oblast that has been under heavy Russian bombardment since the start of the invasion on Feb. 24.

Basurin said they have already started their operation at the Azovstal plant, with Russian forces providing air and artillery support.

The territory of the Azovstal plant is the last holdout for the Ukrainian resistance in Mariupol, as Russian forces accelerate their efforts to capture city. The Mariupol City Council has previously said there are at least 1,000 people, including Ukrainian troops, on the grounds of the plant. Ukrainian Deputy Prime Minister Iryna Vereshchuk said Monday that civilians, including women and children, are also sheltering there and she called for an "urgent humanitarian corridor" to allow them to evacuate.

The Ukrainian government did not immediately confirm Tuesday that Russian forces are storming the Azovstal plant.

-ABC News' Yulia Drozd

Ukrainian military spokesperson Oleksandr Shtupun said Tuesday that Russian forces are stepping up attacks in the east.

Russian forces are currently focused on breaking through Ukrainian defences in the eastern Donetsk and Luhansk oblasts, as well as establishing full control over the strategic port city of Mariupol, according to Shtupun.

In the south, Russian forces are focused on reaching the administrative border of the Kherson Oblast while continuing to shell the city of Mykolaiv, Shtupun said.

Meanwhile, Belarus continues to provide its territory to Russia for reconnaissance and launching air strikes against Ukraine. The Ukrainian cities of Kharkiv, Dnipro, Mykolaiv and Kherson were all shelled overnight, according to Shtupun.

-ABC News' Yulia Drozd