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

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."

Pentagon spokesman John Kirby said Monday that while there has been combat in the Donbas region in eastern Ukraine for some weeks, it is part of Russia’s ongoing "shaping" operations for a future offensive, and not the offensive itself.

"We're not disputing that there's not combat going on in the Donbas," Kirby said. "What we're saying is that we still consider that what we're seeing to be a piece of shaping operations."

“That the Russians are continuing to set conditions for what they believe will be eventual success on the ground by using, by putting, in more forces, putting in more enablers, putting in more command and control capability for operations yet to come," he said.

Asked to clarify his answer, Kirby replied: "We believe that the Russians are shaping and setting the conditions for future offensive operations. We also see ... that there is active combat going on right now in the Donbas as there has been for the last several weeks."

Kirby described the fighting in the besieged port city of Mariupol as part of that since the Russians are "trying to set the conditions for more aggressive, more overt and larger ground maneuvers in the Donbas."

He added that the Russians have also continued to flow in artillery, helicopters, enabling troops and more command and control units as part of the groundwork for that upcoming operation.

Kirby said that the U.S. believes that Russia has reinforced the number of battalion tactical groups in eastern and southern Ukraine as part of their preparations for a large operation in the Donbas region.

-ABC News' Conor Finnegan, Luis Martinez

The U.S. is still assessing what the Russians were attempting to strike in Lviv in western Ukraine on Monday, according to Pentagon spokesman John Kirby.

Kirby said he would not describe Monday's airstrike as a "bombardment" and said he did not know Russia's intent.

"We don't have a clear sense of battle damage assessment about what they were targeting and what they hit. At this time, we don't have any indication that Western aid was targeted and/or hit or destroyed," Kirby said.

-ABC News' Luis Martinez