Ukrainian and Russian forces engaged in heavy fighting Thursday in the battle for the sprawling steel mill that represents the last holdout in the beleaguered city of Mariupol, as signs pointed to an increasingly dire situation for the resistance.

Kateryna Prokopenko, the wife of Azov Regiment commander Denys Prokopenko, a leader of the Azovstal steel plant’s defenders, said he told her in a call from inside that he would love her forever. “It seemed like words of goodbye,” she said.

Earlier Thursday, the deputy commander of the regiment, Capt. Sviatoslav Palamar, pleaded for the evacuation of civilians in a video statement from a bunker and said “wounded soldiers are dying in agony” due to the lack of medical care.

The bloody battle came amid growing speculation that President Vladimir Putin wants to present the Russian people with a battlefield triumph — or announce an escalation of the war — in time for Victory Day on Monday. That is the biggest patriotic holiday on the Russian calendar, marking the Soviet Union’s triumph over Nazi Germany.

The estimated 2,000 Ukrainian fighters and a few hundred civilians remaining at the steel mill after weeks of ferocious bombardment were betrayed by an electrician who showed Russian troops the underground tunnels that lead to the factory, said Anton Gerashchenko, an adviser to Ukraine’s Internal Affairs Ministry. Russian forces first breached the compound Tuesday.

“Yesterday, the Russians started storming these tunnels, using the information they received from the betrayer,” Gerashchenko said in a video posted late Wednesday.

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Latest developments:

►The U.N. food aid agency is appealing for Black Sea ports in Ukraine, blasted by Russian missiles and artillery, to be opened to permit shipping of wheat and corn exports, which many poor nations depend on. Corn and wheat prices have ballooned since the war began.