Russia on Saturday resumed its assault on the last Ukrainian defenders holed up in a giant steel works in Mariupol, a Ukrainian official said, days after Moscow declared victory in the southern port city and said its forces did not need to take the factory.

Reuters quoted presidential adviser Oleksiy Arestovych and said that Russian forces were hitting the Azovstal complex with air strikes and trying to storm it, adding “the enemy is trying to strangle the final resistance of Mariupol’s defenders”.

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The biggest battle of the conflict has raged for weeks as Russia seeks to capture a city seen as crucial to its attempts to link the eastern Donbas region with Crimea, the Black Sea peninsula that Moscow seized in 2014.

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On Friday, Russia’s defence ministry said the remaining fighters had been “securely blockaded” at the steel plant. 

On Thursday, President Vladimir Putin had declared the city “liberated” and ordered his defence minister to block off the Azovstal complex “so not even a fly can get through” rather than try to storm it.

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“There’s no need to climb into these catacombs and crawl underground through these industrial facilities,” Putin said on television.

Arestovych said Ukrainian troops in the Azovstal complex were still holding out “despite the very difficult situation” and were attempting counterattacks.

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More than 1,000 civilians are holed up with the troops at the Azovstal plant, according to Ukrainian authorities.

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Russia has said the invasion it launched on February 24 is a “special military operation” to demilitarise Ukraine and liberate its population from dangerous nationalists. Moscow has denied targeting civilians.