The deadline issued by Moscow Tuesday has passed after it spoke directly to thousands of Ukrainian fighters holed up in a Mariupol steel factory, telling them to surrender or die.

"All who lay down their arms are guaranteed to remain alive," the defense ministry said as it prepares for a major offensive in the southeastern port city.

Donetsk Ukraine

An armed serviceman of Donetsk People's Republic militia walks past a building damaged during fighting in Mariupol on Wednesday. (AP/Alexei Alexandrov)

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The Kremlin gave the Ukrainian fighters a deadline for noon local time – 5 a.m. EDT – telling them to leave "without exception, without any weapons and without ammunition," first reported Reuters. 

The Azovstal Iron and Steel Works was once the site of one of the largest metallurgical factories in Europe, but since the onslaught of the war more than 50 days ago it has become a symbol of the city’s resistance.

The plant’s network of underground tunnels has become home to Mariupol residents seeking shelter from the barrage of shelling and a base for several military units.

Despite weeks of intense shelling that has resulted in the death of more than 20,000 Mariupol residents according to Mayor Vadym Boychenko, Russian forces have been unable to take the strategically important city.

A few thousand Ukrainian troops, by Russia's estimate, remained holed up at a mammoth steel mill in Mariupol, the last known pocket of resistance in the devastated southern port city. (AP)

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Advisor to the Russian-backed separatist forces in the Donetsk region said the Azovstal tunnels were a leading contributor to Russia’s failed attempts to completely besiege the city, claiming they could withstand "even [a] nuclear strike."

Boychenko said last week that several military units were still fighting in Mariupol, including the 36th Marine Brigade, Interior Ministry troops, border guards and the National Guard’s controversial Azov Regiment.

On Monday commander of the Azov Regiment, Denys Prokopenko, said that Russian forces were dropping bunker-buster bombs on the plant.

Smoke rises above Azovstal steelworks, in Mariupol, Ukraine, in this still image obtained from a recent drone video posted on social media.  (MARIUPOL CITY COUNCIL/via REUTERS  THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES.)

U.S. defense officials maintain that Mariupol is still a contested city, but Russia’s blockade on the city has many concerned Ukraine will be hard-pressed to resupply its fighters. 

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Russia announced Tuesday that its second phase in the war in Ukraine had begun after security officials have been sounding the alarm for weeks that Moscow will hit the easternmost regions with a "major offensive."

"No matter how many soldiers are drawn there, we will defend ourselves," Ukrainian President Volodymyr Zelenskyy pledged Monday night. "We will fight. We will not give up anything Ukrainian."

The Associated Press contributed to this report.