Russia's defence ministry on Friday said the last group of Ukrainian forces holed up in Mariupol's smashed Azovstal steel works had surrendered, marking an end to a weeks-long siege.

"The territory of the Azovstal metallurgical plant ... has been completely liberated," the ministry said in a statement. It said the group that had given up comprised 531 people.

The full abandonment of the bunkers and tunnels of the bombed-out plant means an end to most destructive siege of a war that began when Russia attacked Ukraine nearly three months ago.

Ukraine ordered the garrison to stand down on Monday. Hours before the Russian announcement, President Volodymyr Zelenskiy said the defenders had been told by Ukraine's military that they could get out and save their lives and would most likely all leave in the coming days.

"The underground facilities of the enterprise, where the militants were hiding, came under the full control of the Russian armed forces," said the Russian statement, adding that a total of 2,439 defenders had surrendered in the past few days.

Defence Minister Sergei Shoigu has told President Vladimir Putin that both Mariupol and the steel plant have been fully liberated, it said. (Reuters)