(NEW YORK) — Russia launched an all-out ground offensive to take control of eastern Ukraine late Monday, marking the long-feared start of a new phase of the nearly two-month-long war.

Ukrainian officials said Russian forces were attacking along a nearly 300-mile front in the disputed Donbas region, the predominately Russian-speaking industrial heartland of Ukraine’s east, where Russia-backed separatists have been fighting Ukrainian soldiers since 2014 and have declared two independent republics recognized by Moscow.

“Russian troops have begun the battle for Donbas, for which they have been preparing for a long time,” Ukrainian President Volodymyr Zelenskyy announced late Tuesday in his nightly broadcast. “A very large part of the entire Russian army is now focused on this offensive.”

Since invading neighboring Ukraine on Feb. 24 from Belarus, to the north, and Russia, to the east, Russian forces have struggled to take full control of major cities amid strong resistance from Ukrainian troops. After failing to seize the capital, Kyiv, Russian forces retreated from northern Ukraine and have been regrouping in the east in recent weeks in preparation for a full-scale assault, as Russian officials declared “liberating” the Donbas as the main goal of the “special military operation.”

On Tuesday morning, Russian Foreign Minister Sergey Lavrov announced that “another stage of this operation is beginning.”

“I am sure this will be a very important moment of this entire special operation,” Lavrov said in an interview with India Today, an English-language Indian television network.

Lavrov noted that the objective, “as it was declared from the very start,” was to “fully liberate” the self-proclaimed Donetsk and Luhansk people’s republics in the Donbas.

The Russian military called on the outnumbered Ukrainian fighters in the besieged port city of Mariupol in eastern Ukraine’s Donetsk Oblast to surrender by midday. The Russian Ministry of Defense said it was ready to declare a temporary ceasefire in Mariupol on Tuesday from 1:30 p.m. local time.

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

“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 would have been allowed to leave the plant without any weapons or ammunition from 2 p.m. to 4 p.m. local time. Moscow guaranteed to each Ukrainian soldier who surrenders that their life will be spared and their rights as prisoners of war will be respected, Mizintsev said.

The territory of the giant Azovstal plant is the last holdout for the Ukrainian resistance in Mariupol. 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, were also sheltering there. She called for an “urgent humanitarian corridor” to allow them to evacuate.

Eduard Basurin, a spokesperson for the militia of the self-declared Donetsk People’s Republic, told Russian state media on Tuesday that separatist special forces were chosen to assist the Russian military in storming the Azovstal plant. Basurin said they have already started the operation in Mariupol, with Russian forces providing air and artillery support.

The Ukrainian government did not immediately confirm the claim. However, Ukrainian military spokesperson Oleksandr Shtupun said Tuesday that Russian forces were intensifying their attacks in the east, with a focus on breaching Ukrainian defenses in the Donetsk and Luhanks oblasts, as well as establishing full control of Mariupol.

Weeks of relentless Russian bombardment have largely reduced Mariupol to rubble and killed thousands of residents, according to local officials. Capturing the city — a strategically important port on the Sea of Azov, part of the Black Sea — is the remaining obstacle to Russia’s push to secure a a coastal corridor to Ukraine’s Crimean Peninsula, which Russian forces invaded and subsequently annexed in 2014. Earlier this month, NATO Secretary-General Jens Stoltenberg warned that a full-scale ground offensive by Russia in Ukraine’s east would be “a crucial phase of the war.”

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