LVIV, Ukraine (AP) — Ukrainian fighters battling Russian forces in the tunnels beneath Mariupol’s immense steel plant refused to surrender in the face of relentless attacks, with the wife of one commander saying they had vowed to “stand till the end.”

The fight in the last Ukrainian stronghold of the strategic port city reduced to ruins by the Russian onslaught appeared increasingly desperate 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.

“They won’t surrender,” Kateryna Prokopenko said Thursday after speaking by phone to her husband, a leader of the steel plant defenders. “They only hope for a miracle.”

She said her husband, Azov Regiment commander Denys Prokopenko, told her he would love her forever. “I am going mad from this. It seemed like words of goodbye,” she said.

The Ukrainian military’s General Staff said Friday that “the blockade of units of the defense forces in the Azovstal area continues” and that the Russians, with aviation support, had resumed assault operations to take control of the sprawling plant.

Monday’s Victory Day is the biggest patriotic holiday on the Russian calendar, marking the Soviet Union’s triumph over Nazi Germany. But as long as Ukrainians resist the takeover of the plant, “Russian losses will continue to build and frustrate their operational plans in southern Donbas,” the British Defense Ministry said in an assessment.

Some 2,000 Ukrainian fighters, by Russia’s most recent estimate, were holed up in a maze of tunnels and bunkers beneath Azovstal steelworks. A few hundred civilians were also believed trapped there.

“There are many wounded (fighters), but they are not surrendering,” Ukrainian President Volodymyr Zelenskyy said in his nightly video address. “They are holding their positions.”

“Just imagine this hell! And there are children there,” he said. “More than two months of constant shelling, bombing, constant death.”

The Russians managed to get inside the plant Wednesday with the help of an electrician who knew the layout, said Anton Gerashchenko, an adviser to Ukraine’s Internal Affairs Ministry.

“He showed them the underground tunnels which are leading to the factory,” Gerashchenko said in a video.

Zelenskyy said the attack was preventing evacuation of the remaining civilians, even as U.N. Secretary-General Antonio Guterres said another attempt was underway. “We must continue to do all we can to get people out of these hellscapes,” Guterres said.

The Kremlin denied its troops were storming the plant and has demanded the Ukrainians surrender. They have refused. Russia has also accused the fighters of preventing the civilians from leaving.

The fall of Mariupol would deprive Ukraine of a vital port, allow Russia to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014, and free up troops to fight elsewhere in the Donbas, the eastern industrial region that the Kremlin says is now its chief objective.

Capt. Sviatoslav Palamar, deputy commander of the Azov Regiment, pleaded on Ukrainian TV for the evacuation of civilians and wounded fighters from the steelworks, saying soldiers were “dying in agony due to the lack of proper treatment.”

More than 100 civilians were rescued from the steelworks over the weekend. But many previous attempts to open safe corridors from Mariupol have fallen through, with Ukraine blaming shelling and firing by the Russians.

Meanwhile, 10 weeks into the devastating war, Ukraine’s military claimed it recaptured some areas in the south and repelled other attacks in the east, further frustrating Putin’s ambitions after his abortive attempt to seize Kyiv. Ukrainian and Russian forces are fighting village by village.

The General Staff in Kyiv said Russian forces were conducting surveillance flights, and in the hard-hit areas of Donetsk and Luhansk, Ukrainian forces repulsed 11 attacks and destroyed tanks and armored vehicles. Russia gave no immediate acknowledgement of those losses.

Pentagon spokesman John Kirby said Russian forces are making only “plodding” progress in the Donbas.

There are growing suggestions that Ukraine might try to widen its push to seize more territory from Russia outside Kharkiv, its second-largest city.

Ukrainian chief of defense, Gen. Valerii Zaluzhnyi, said Thursday that a counteroffensive could begin to push Russian forces away from Kharkiv and Izyum, which has been a key node in Russia’s control of the eastern cauldron. Ukraine in recent days has driven Russian troops some 40 kilometers (25 miles) east of Kharkiv, which has been repeatedly struck by Russian shelling.

Additional Ukrainian advances may spare the city from artillery strikes, as well as force Moscow to divert troops from other areas of the front line.

On Thursday, an American official said the U.S. shared intelligence with Ukraine about the location of a Russian flagship before the mid-April strike that sank it, one of Moscow’s highest-profile failures in the war.

The U.S. has provided “a range of intelligence” that includes locations of warships, said the official, who was not authorized to speak publicly and spoke on condition of anonymity. The official said the decision to target the missile cruiser Moskva was purely a Ukrainian decision.

Fearful of new attacks surrounding Victory Day, the mayor of the western Ukrainian city of Ivano-Frankivsk urged residents to leave for the countryside over the long weekend and warned them not to gather in public places.

And the southeastern city of Zaporizhzhia, a key transit point for evacuees from Mariupol, announced a curfew from Sunday evening through Tuesday morning.

Mariupol, which had a prewar population of over 400,000, has come to symbolize the misery inflicted by the war. The siege of the city has trapped perhaps 100,000 civilians with little food, water, medicine or heat.

As the battle raged there, Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said Russian bombardment Thursday hit dozens of Ukrainian military targets, including troop concentrations in the east, an artillery battery near the eastern settlement of Zarozhne and rocket launchers near the southern city of Mykolaiv.

The war has devastated Ukraine’s medical infrastructure, Zelenskyy said in a video link to a charity event in the U.K. Nearly 400 health care facilities have been damaged or destroyed, he said.

“There is simply a catastrophic situation regarding access to medical services and medicines,” in areas occupied by Russian forces, he said. “Even the simplest drugs are lacking.”

With the challenge of mine-clearing and rebuilding after the war in mind, Zelenskyy announced the launch of a global fundraising platform called United24.

At the same time, Poland hosted an international donor conference that raised $6.5 billion in humanitarian aid. The gathering was attended by prime ministers and ambassadors from many European countries, as well as representatives of other nations and some businesses.

In addition, a Ukrainian cabinet body began to develop proposals for a comprehensive postwar reconstruction plan, while Zelenskyy also urged Western allies to put forward a program similar to the post-World War II Marshall Plan plan to help Ukraine rebuild.