Russia said on Wednesday nearly 700 more Ukrainian fighters had surrendered in Mariupol, but Kyiv was silent about their fate, while a pro-Russian separatist leader said commanders were still holed up in tunnels beneath the giant Azovstal steelworks.

More than a day after Kyiv announced it had ordered its garrison in Mariupol to stand down, Ukrainian officials halted all public discussion of the fate of remaining fighters who had made their last stand there.

Russia’s ministry of defence said 694 more fighters had surrendered overnight, bringing the total number of people who had laid down arms to 959. Ukraine’s defence ministry did not respond to requests 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 had yet to surrender: “They have not left”, DAN news agency quoted Denis Pushilin as saying.

Ukrainian officials had confirmed the surrender of more than 250 fighters on Tuesday. But they did not say how many more were inside or what would become of them, and made clear on Wednesday that there would be no further comment for now.

“Unfortunately, the subject is very sensitive and there is a very fragile set of talks going on today, therefore I cannot say anything more,” said Mariupol Mayor Vadym Boichenko. He said President Volodymyr Zelenskiy, the Red Cross and the United Nations were involved in talks, but gave no further details.

“The main thing is that our boys saved our country, gave us a chance to get ready for this destructive war.”

The negotiations over the surrender of Mariupol came as Finland and Sweden 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.

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

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 re-opened prison, both in towns held by pro-Russian separatists. Reuters journalists have filmed buses bringing captured fighters to both locations.

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.

Finland and Sweden apply to NATO

The Swedish and Finnish ambassadors handed over their NATO membership application letters in a ceremony at the alliance's headquarters in Brussels.

“This is a historic moment which we must seize,” NATO Secretary-General Jens Stoltenberg said.

Ratification of all 30 allied parliaments could take up to a year, diplomats say. Turkey has surprised its allies in recent days by saying it had reservations about the new prospective members, especially their tolerance of Kurdish militant groups on their soil.

Stoltenberg said he thought the issues could be overcome. Washington has also played down the likelihood that Turkish objections would halt the accession.

Finland, which shares a 1,300-km (810-mile) border with Russia, and Sweden were both militarily non-aligned throughout the Cold War, and their decision to join the alliance represents the biggest change in European security for decades.

In a stroke, it will more than double the alliance's land border with Russia, give NATO control over nearly the entire coast of the Baltic Sea and put NATO guards just a few hours' drive north of St Petersburg.

After weeks in which Russia threatened retaliation against the plans, Putin appeared to abruptly climb down this week, saying in a speech on Monday that Russia had "no problems" with either Finland or Sweden, and their NATO membership would not be an issue unless the alliance sent more troops or weapons there.

Victory

The steelworks surrender in Mariupol would let Putin claim a rare victory in a campaign which has otherwise faltered. Recent weeks have seen Russian forces abandon the area around Ukraine's second larges 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 Donbas region of southeastern Ukraine which it claims on behalf of separatists it has supported since 2014.

Mariupol, the main port for the Donbas, 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 Europe's deadliest battle at least since 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 centres.

Russia, which denies targeting civilians, insists it had agreed to no prisoner swap in advance for the Azovstal defenders. Many of the fighters 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,” Russian Deputy Ambassador to the United Nations Dmitry Polyansky tweeted.

Russia’s state-owned TASS news agency reported a Russian committee planned to question the soldiers as part of an investigation into what Moscow calls “Ukrainian regime crimes”.