More than 1,000 Ukrainian marines have surrendered in the port of Mariupol, Russia’s defence ministry said on Wednesday of its main strategic target in the eastern Donbas region, which has been reduced to ruins but is not yet under Russian control.

If the Russians take the Azovstal industrial district, where the marines have been holed up, they would be in full control of Mariupol, Ukraine’s main Sea of Azov port, allowing Russia to reinforce a land corridor between separatist-held eastern areas and the Crimea region that it seized and annexed in 2014.

Surrounded and bombarded by Russian troops for weeks and the focus of some of the fiercest fighting of the war, Mariupol would be the first major city to fall since Russia invaded Ukraine on February 24. Russia’s defence ministry said that 1,026 marines had surrendered, including 162 officers.

“In the town of Mariupol, near the Ilyich Iron and Steel Works, as a result of successful offensives by Russian armed forces and Donetsk People’s Republic militia units, 1,026 Ukrainian soldiers of the 36th Marine Brigade voluntarily laid down arms and surrendered,” the ministry said in a statement.

Ukraine’s general staff said Russian forces were proceeding with attacks on Azovstal and the port, but a defence ministry spokesman said he had no information about any surrender.

Reuters journalists accompanying Russian-backed separatists saw flames billowing from the Azovstal district on Tuesday.

On Monday, the 36th Marine Brigade said it was preparing for a final battle in Mariupol that would end in death or capture as its troops had run out of ammunition.

Thousands of people are believed to have been killed in Mariupol and Russia has been massing thousands of troops in the area for a new assault, Ukrainian President Volodymyr Zelensky said.

Ukraine accuses Russia of blocking aid convoys to tens of thousands of civilians is says are still trapped in the city.

Chechen leader Ramzan Kadyrov, an ardent supporter of Russian President Vladimir Putin, urged remaining Ukrainians holed up in Azovstal to surrender.

“Within Azovstal at the moment there are about 200 wounded who cannot receive any medical assistance,” Kadyrov said in a Telegram post. “For them and all the rest it would be better to end this pointless resistance and go home to their families.”

Russian television showed pictures of what it said were marines giving themselves up at Illich Iron and Steel Works in Mariupol on Tuesday, many of them wounded.

Ukrainian deputy defence minister Hanna Malyar has said there was a high risk of Russia using chemical weapons in their assault on the country, echoing earlier warnings by Zelensky, who on Wednesday told the Estonian parliament by videolink Russia was using phosphorus bombs to terrorise civilians.

He did not provide evidence and Reuters has not been able to independently verify his assertion.

Chemical weapons production, use and stockpiling is banned under the 1997 Chemical Weapons Convention. White phosphorous, although condemned by human rights groups, is not banned.