Nearly 700 more Ukrainian fighters at a giant steel mill in Mariupol have surrendered in the past 24 hours, Russia said Wednesday, as a top Ukrainian military official hoped they could be exchanged for Russian prisoners of war.

Finland and Sweden, meanwhile, formally applied to join NATO, bringing about the very expansion that Russian President Vladimir Putin has long cited as one of his main reasons for launching the "special military operation" in February.

Ukraine ordered the fighters to save their lives — and said their mission to tie up Russian forces is now complete — but has not called the column of soldiers walking out of the plant a surrender. The fighters face an uncertain fate, with Ukraine saying they hope for a prisoner swap but Russia vowing to put at least some of them on trial for war crimes.

It is not clear how many fighters remain inside the last stronghold, Ukraine’s city now largely reduced to rubble. Both sides are trying to shape the narrative and extract propaganda victories from what has been one of the most important battles of the war.

Russian Defense Ministry spokesperson Maj. Gen. Igor Konashenkov said Wednesday that 959 Ukrainian troops have now abandoned the Avozstal plant since they started coming out Monday. At one point, officials put the number of fighters holed up in the mill’s sprawling network of tunnels and bunkers at 2,000.

If confirmed, the Russian announcement would resolve much of the mystery surrounding the fate of hundreds of fighters inside the plant, since Ukraine announced on Tuesday it had ordered the entire garrison to stand down. The Ukrainian Defense Ministry, which has so far confirmed only about 250 have left the plant, did not immediately respond to a written request for comment.

The leader of pro-Russian separatists in control of the area was quoted by a local news agency as saying the main commanders inside the plant are yet to surrender: "They have not left," DAN news agency quoted Denis Pushilin as saying.

The final surrender of Mariupol would bring a close to a nearly three-month siege of the once prosperous city of 400,000 people, where Ukraine says tens of thousands of civilians died under Russian siege and bombardment, many buried in mass graves.

Kyiv and Moscow had both said on Tuesday that around 250 people left the plant, giving little clue as to the fate of hundreds more believed to be inside. Ukraine said it would not reveal how many were there until the operation to rescue all of them was complete.

Ukrainian officials have spoken of hopes to arrange a prisoner swap for Mariupol defenders they describe as national heroes. Moscow says no such deal was made for fighters it calls "Nazis."

Russia says more than 50 wounded fighters have been brought for treatment to a hospital, and others have been taken to a newly reopened prison, both in towns held by pro-Russian separatists.

The Kremlin says Putin has personally guaranteed the humane treatment of those who surrender, but high-profile Russian politicians have publicly called for them never to be exchanged, or even for their execution.

Russia not in favor of swap

Russia insists it had agreed to no prisoner swap in advance for the Azovstal defenders, many of whom belong to the Azov Regiment, a Ukrainian unit with origins as a far-right militia, which Russia describes as Nazis and blames for mistreating Russian speakers.

"I didn’t know English has so many ways to express a single message: the #Azovnazis have unconditionally surrendered," tweeted Russian Deputy Ambassador to the United Nations Dmitry Polyansky.

TASS news agency reported a Russian committee planned to question the soldiers as part of an investigation into what Moscow calls "Ukrainian regime crimes."

Leonid Slutsky, one of Russia's negotiators in talks with Ukraine, called the evacuated combatants "animals in human form" and said they should be executed.

The steelworks' surrender in Mariupol allows Putin to claim a rare victory in a campaign that has otherwise faltered. Recent weeks have seen Russian forces abandon the area around Ukraine's second-largest city Kharkiv, now retreating at their fastest rate since they were driven from the north and the Kyiv environs at the end of March.

Nevertheless, Moscow has continued to press on with its main offensive, trying to capture more territory in the Donbass region of southeastern Ukraine, which it claims on behalf of separatists it has supported since 2014.

Mariupol, the main port for the Donbass, is the biggest city Russia has captured so far, and gives Moscow full control of the Sea of Azov and an unbroken swathe of territory across the east and south of Ukraine.

The siege was the deadliest battle in Europe at least since the wars in Chechnya and the Balkans of the 1990s.

The city's months of resistance became a global emblem of Ukraine's refusal to yield against a far better-armed foe, while its near-total destruction demonstrated Russia's tactic of raining down fire on population centers.

Battle for Donbass

The battle for Mariupol is Russia's biggest victory since its invasion, giving it control of the Azov Sea coast and an unbroken stretch of eastern and southern Ukraine. But the port lies in ruins, and Ukraine believes tens of thousands of people were killed under months of Russian bombardment.

Russia's offensive in the east, meanwhile, appeared to be making little progress, although the Kremlin says all its objectives will be reached in its bid to "demilitarise" Ukraine.

Ukraine's military command said Russia continued to shell Ukrainian positions along the entire frontline in the east on Wednesday.

Around a third of the Donbass was held by Russia-backed separatists before the invasion. Moscow now controls around 90% of the Luhansk region, but it has failed to make major inroads toward the key cities of Sloviansk and Kramatorsk in Donetsk in order to extend control over the entire Donbass.

Ukrainian forces have advanced at their fastest pace for more than a month, driving Russian forces out of the area around Kharkiv, Ukraine's second-largest city.