Russian forces have resumed their assault on a sprawling steel factory in the devastated Ukrainian port of Mariupol, using aircraft to pound Ukrainian fighters holding out there.

As many as 2,000 Ukrainian fighters, plus hundreds of civilians, were believed to remain on the grounds of the Azovstal plant, whose defenses were breached earlier this week by Russian forces.

The United Nations said another evacuation effort was under way.

Ukraine’s General Staff said in its daily assessment on May 6 that Russians were using aircraft as part of the renewed assault on the plant.

“There are many wounded, but they are not surrendering,” Ukrainian President Volodymyr Zelenskiy said on May 5 in his nightly video address. “They are holding their positions.”

Mariupol itself has been largely razed to the ground by weeks of street-to-street fighting and heavy bombardment. Azovstal has turned into a last stand for the Ukrainians troops struggling to prevent a complete Russian defeat of the city.

The fighting comes as Russia continues its offensive in the eastern Donbas, an offensive that has proceeded slowly and without major advances, as Ukrainian forces have blocked Russian movements and even regained territory.

The fight for Azovstal also comes amid speculation that Russian President Vladimir Putin wants a battlefield triumph that he can showcase on May 9 when Russia marks Victory Day — the anniversary of the defeat of Nazi Germany.

“The renewed effort by Russia to secure Azovstal and complete the capture of Mariupol is likely linked to the upcoming 9 May Victory Day commemorations and Putin’s desire to have a symbolic success in Ukraine,” the British Defense Ministry said in its May 6 daily assessment.

“This effort has come at personnel, equipment, and munitions cost to Russia. Whilst Ukrainian resistance continues in Azovstal, Russian losses will continue to build and frustrate their operational plans in southern Donbas,” the ministry said.

Losing Mariupol would deprive Ukraine of a vital port on the Sea of Azov. It would also give Russia the ability to establish a land corridor to the Crimean Peninsula and free up troops to fight elsewhere in the Donbas.