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

The Russian military is calling on Ukrainian troops in besieged Mariupol to surrender by midday on Tuesday.

Russian forces have been trying to seize the strategic port city in eastern Ukraine's war-torn Donetsk Oblast since the start of the invasion on Feb. 24. The Russian Ministry of Defense said it is ready to declare a temporary ceasefire in Mariupol on Tuesday from 1:30 p.m. local time.

Ukrainian troops holed up on the grounds of the Azovstal iron and steelworks plant in Mariupol have until 12 p.m. local time to stop all fighting and lay down their arms, according to Col. Gen. Mikhail Mizintsev, head of Russia's National Defense Control Center. Moscow has proposed this plan "given the catastrophic situation in the Azovstal Iron and Steel Works, as well as for purely humanitarian considerations," Mizintsev told Russian state media on Tuesday.

"All those who will lay down arms are guaranteed that their lives will be spared," he added. "The actual start of the temporary ceasefire shall be marked by both sides by raising flags -- red flags by the Russian side and white flags by the Ukrainian side along the entire perimeter of Azovstal. Furthermore, their readiness to put the temporary ceasefire into effect shall be confirmed by the sides via all communication channels."

According to Mizintsev, "absolutely all" Ukrainian troops and foreign mercenaries will be allowed to leave the plant without any weapons or ammunition from 2 p.m. to 4 p.m. local time. Moscow guarantees to each Ukrainian soldier who surrenders that their life will be spared and their rights as prisoners of war will be respected, Mizintsev said.

Ukrainian President Volodymyr Zelenskyy said Russian forces have begun their offensive in the eastern part of the country after a week of building up troops.

"It can now be stated that Russian troops have begun the battle for Donbas, for which they have been preparing for a long time," the president said in his nightly address. "A very large part of the entire Russian army is now focused on this offensive."

Ukrainian and U.S. officials have said Russian troops exiting the Kyiv region over the last week were moving to eastern Ukraine as part of a new effort to take over land partially controlled by the self-proclaimed Republics of Donetsk and Luhansk. Those two breakaway regions were already aligned with Russia following a war that began in 2014.

Zelenskyy, however, warned that Russian forces would not be successful in taking all of the land in the southeastern region of Ukraine.

"No matter how many Russian soldiers are driven there, we will fight," he said. "We will defend ourselves. We will do it daily. We will not give up anything Ukrainian, and we do not need what's not ours."

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan