More than 250 Ukrainian fighters surrendered to Russian forces at the Azovstal steel works in Mariupol after weeks of desperate resistance, bringing an end to the most devastating siege of Russia's war in Ukraine and allowing President Vladimir Putin to claim a rare victory in his faltering campaign.

Even as the Kremlin prepares to take full control of the ruins of Mariupol, it faces the growing prospect of defeat in its bid to conquer all of Ukraine's eastern Donbas because its badly mauled forces lack the manpower for significant advances, some analysts of the Russian campaign said.

Reuters saw buses leave the steelworks late yesterday in a convoy escorted by Russian armoured vehicles. Five arrived in the Russian-held town of Novoazovsk, where Moscow said the wounded would be treated.

What will happen to the fighters was unclear. The Kremlin said Vladimir Putin had personally guaranteed the prisoners would be treated according to international standards, and Ukrainian officials said they could be exchanged for Russian captives.

Service members of pro-Russian troops stand guard near the besieged Azovstal steel mill

A Reuters witness said seven buses carrying Ukrainian fighters from the Azovstal garrison arrived at a newly reopened prison in the Russian-controlled town of Olenivka near Donetsk.

TASS news agency said a Russian committee planned to question the soldiers, many of them members of the Azov Battalion, as part of an investigation into what Moscow calls "Ukrainian regime crimes".

The denouement of a battle which came to symbolise Ukrainian resistance gives Moscow total control of the Azov Sea coast and an unbroken stretch of eastern and southern Ukraine, even as its troops retreat from the outskirts of Kharkiv in the northeast.

On the international front, Swedish Prime Minister Magdalena Andersson said Sweden and Finland will hand in their respective applications to join NATO tomorrow, abandoning their long-standing policy of neutrality over concerns about Mr Putin's wider intentions.

Their move will bring about the very expansion of the Western alliance Mr Putin invoked as one of the main justifications for what he terms his "special military operation".

Prisoner swap

The complete capture of Mariupol is Russia's biggest victory since its 24 February invasion. But the port lies in ruins, and Ukraine believes tens of thousands of people were killed under months of Russian bombardment.

Russia said at least 256 Ukrainian fighters had "laid down their arms and surrendered," including 51 severely wounded. Ukraine said 264 soldiers, including 53 wounded, had left.

Mariupol is in ruins following Russia's invasion of Ukraine

Russian defence ministry video showed fighters leaving the plant, some carried on stretchers, others with hands up to be searched by Russian troops.

While both sides spoke of a deal under which all Ukrainian troops would abandon the steelworks, many details were not yet public, including how many fighters still remained inside, and whether any form of prisoner swap had been agreed.

Ukraine's Deputy Defence Minister Hanna Malyar told a briefing that Kyiv would not disclose how many fighters remained inside until all were safe. Ukraine's military said units in Azovstal had completed their combat mission.

Ukraine's Deputy Prime Minister Iryna Vereshchuk said Kyiv aimed to arrange a prisoner swap for the wounded once their condition stabilises, but neither side disclosed terms for any specific deal.

High-profile Russian lawmakers spoke out against any prisoner swap. Vyacheslav Volodin, speaker of the State Duma, Russia's lower house, said: "Nazi criminals should not be exchanged."

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

A Ukrainian police officer views the destruction at the "Barabashovo" clothing market in Kharkiv

Natalia, wife of a sailor among those holed up in the plant, told Reuters she hoped "there will be an honest exchange". But she was still worried: "What Russia is doing now is inhumane".

The United Nations and Red Cross say the true death toll from the Mariupol siege is still uncounted but it is certain to be Europe's worst since the 1990s wars in Chechnya and the Balkans.

For months, residents were driven into cellars under perpetual bombardment, with no access to food, fresh water or heat, and bodies littering the streets.

Two strikes - on a maternity ward and a theatre where hundreds of people were sheltering - became worldwide emblems of Russia's tactic of devastating population centres.

Thousands of civilians are believed to have been buried in mass graves or makeshift pits in gardens, and Ukraine says Moscow forcibly deported thousands of residents to Russia.

Moscow denies targeting civilians or deporting them.

We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences

Ukrainian advances

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

Ukraine says its forces had reached the Russian border, 40km north of Kharkiv. They have also pushed at least as far as the Siverskiy Donets river 40km to the east, where they could threaten supply lines to Russia's main advance in the Donbas.

Russia is still pressing that advance despite taking heavy losses.

Also today, Ukrainian President Volodymyr Zelenskiy spoke by phone to French President Emmanuel Macron, who told him that French arms deliveries to Ukraine would intensify in the coming days and that France was ready to respond to additional demands for help.

The US Senate is moving closer to advancing a $40 billion aid package for Ukraine, including billions for new weapons. A final vote on the package is possible later this week.

Mr Putin may have to decide whether to send in more troops and hardware to replenish his dramatically weakened invasion force as an influx of modern Western weaponry bolsters Ukraine's combat power, analysts said.

"Time is definitely working against the Russians. They're running out of equipment. They're running out of particularly advanced missiles. And, of course, the Ukrainians are getting stronger almost every day," said Neil Melvin of the RUSI think-tank in London.