Mariupol appeared on the verge of falling to the Russians on Tuesday as Ukraine moved to abandon the steel plant where hundreds of its fighters had held out for months under relentless bombardment in the last bastion of resistance in the devastated city.

The capture of Mariupol would make it the biggest city to be taken by Moscow’s forces in the war yet and would give the Kremlin a badly needed victory, though the landscape has largely been reduced to rubble.

More than 260 Ukrainian fighters, some of them seriously wounded and taken out on stretchers, left the ruins of the Azovstal plant on Monday and turned themselves over to the Russian side in a deal negotiated by the warring parties.

Ukrainian authorities said they were working to extract the remaining soldiers from the sprawling steel mill. They would not say how many were still there.

While Russia called it a surrender, the Ukrainians avoided that word and instead said the plant’s garrison had successfully completed its mission to tie down Russian forces and was under new orders.

Ukraine Russia Mariupol
Ukrainian servicemen after being evacuated from the Azovstal steel plant (AP)

“To save their lives, Ukraine needs them. This is the main thing,” Ukrainian defence minister Oleksiy Reznikov said.

The Ukrainians expressed hope that the fighters would be exchanged for Russian prisoners of war.

But Vyacheslav Volodin, speaker of the lower house of the Russian parliament, said without evidence that there were “war criminals” among the defenders and that they should not be exchanged but tried.

Ukrainian president Volodymyr Zelensky said Ukraine is still working to extract its remaining troops from the sprawling steel mill. Officials have not said how many remain inside.

“The evacuation mission is continuing, it is being supervised by our military and intelligence officers,” Mr Zelensky said in his nightly video address. “The most influential international mediators are involved.”

The operation to abandon the steel plant and its labyrinth of tunnels and bunkers signalled the beginning of the end of a nearly three-month siege that turned Mariupol into a worldwide symbol of both defiance and suffering.

The Russian bombardment killed more than 20,000 civilians, according to the Ukrainian side, and left the remaining inhabitants, perhaps one-quarter of the southern port city’s prewar population of 430,000, with little food, water, heat or medicine.

Ukraine graphic
(PA Graphics)

During the siege, Russian forces launched lethal airstrikes on a maternity hospital and a theatre where civilians had taken shelter. Close to 600 people may have been killed at the theatre.

Gaining full control of Mariupol would give Russia an unbroken land bridge to the Crimean Peninsula, which it seized from Ukraine in 2014, and deprive Ukraine of a vital port.

It could also free up Russian forces to fight elsewhere in the Donbas, the eastern industrial heartland that the Kremlin is bent on capturing.

And it would give Russia a victory after repeated setbacks on the battlefield and the diplomatic front, beginning with the abortive attempt to storm Kyiv, the capital.

Over the past few days, Sweden and Finland announced plans to apply for Nato membership, and Ukraine reported that Moscow’s forces had retreated from around the north-eastern city of Kharkiv in the face of counterattacks and had taken heavy losses in an Ukrainian assault on a pontoon bridge in the Donbas.

Ukraine Russia Mariupol
The operation to abandon the steel plant signalled the beginning of the end of a nearly three-month siege (AP)

Ukrainian presidential adviser Mykhailo Podolyak likened the Ukrainian defenders to the vastly outnumbered Spartans who held out against Persian forces in ancient Greece.

“Eighty three days of Mariupol defence will go down in history as the Thermopylae of the XXI century,” he tweeted.

The soldiers who left the plant were searched by Russian troops, loaded on to buses accompanied by Russian military vehicles, and taken to two towns controlled by Moscow-backed separatists.

More than 50 of the fighters were seriously wounded, according to both sides.

Russia’s main federal investigative body said it intends to interrogate the troops to “identify the nationalists” and determine whether they were involved in crimes against civilians.

Ukraine Russia Mariupol
The Russian bombardment killed more than 20,000 civilians, according to the Ukrainian side (Alexei Alexandrov/AP)

Also, Russia’s top prosecutor asked the country’s supreme court to designate Ukraine’s Azov regiment, whose members have been holding out at Azovstal, a terrorist organisation. The regiment has links to the far right.

A negotiated withdrawal could save lives on the Russian side, too, sparing its troops from what almost certainly would be a bloody battle to finish off the defenders inside the plant, which sprawls over four square miles.

The withdrawal could also work to Moscow’s advantage by taking the world’s attention off the suffering in Mariupol.

Russian and Ukrainian officials said peace talks were on hold.

In other developments, the chief prosecutor for the International Criminal Court prosecutor, Karim Khan, said he sent a team of 42 investigators, forensic experts and support personnel to Ukraine to look into suspected war crimes.

Ukraine has accused Russian forces of torturing and killing civilians.

The World Health Organisation has verified 226 attacks on health facilities in Ukraine, almost three per day on average, since the Russian invasion began, according to the agency’s Europe director, Hans Kluge.

The targeted strikes have killed at least 75 people and wounded 59, he said.

“These attacks are not justifiable, they are never OK and they must be investigated,” he said.