The Russian military has said that more Ukrainian fighters who were making a last stand in Mariupol have surrendered, bringing the total who have left their stronghold to 1,730, while the Red Cross said it had registered hundreds of them as prisoners of war.

The International Committee of the Red Cross said that the registrations of Ukrainian prisoners of war, which included wounded fighters, began on Tuesday under an agreement between Russia and Ukraine.

The Geneva-based humanitarian agency, which has experience in dealing with prisoners of war and prisoner exchanges, said however that its team did not transport the fighters to "the places where they are held" - which was not specified.

Ukrainian fighters who emerged from the ruined Azovstal steelworks after being ordered by their military to abandon the last stronghold of resistance in the now-flattened port city face an uncertain fate.

Some were taken by the Russians to a former penal colony in territory controlled by Moscow-backed separatists.

While Ukraine said it hopes to get the soldiers back in a prisoner swap, Russia threatened to put some of them on trial for war crimes.

The Red Cross cited rules under the Geneva Conventions that should allow the organisation to interview prisoners of war "without witnesses" and that visits with them should not be "unduly restricted".

The organisation did not specify how many prisoners of war were involved.

It was also not clear how many fighters are left at the plant. Russia previously estimated that it had been battling some 2,000 troops in the waterside plant.

Denis Pushilin, a senior Russia-backed separatist official in a region that includes Mariupol, said that those Ukrainian soldiers who needed medical assistance were admitted to hospital while others were put in a detention facility.

He also claimed that Red Cross representatives were allowed to inspect the detention facility, but that could not be immediately verified.

Despite the setback in Mariupol, Ukraine's confidence has been growing after fighting the Russian offensive to an effective standstill and forcing Moscow to withdraw from around Kyiv and narrow its military goals.

Mykhailo Podolyak, an adviser to President Volodymyr Zelensky who was involved in several rounds of talks with Russia, said on Thursday in a tweet that at this stage "do not offer us a ceasefire - this is impossible without total Russian troops withdrawal".

"Until Russia is ready to fully liberate occupied territories, our negotiating team is weapons, sanctions and money," he tweeted.

Ukraine's military said in its morning briefing on Thursday that Russian forces were still pressing their offensive on various sections of the front in the east, but were being successfully repelled.

In the eastern Donbas region, which has been the centre of recent fighting as Russian forces on the offensive have clashed with staunch Ukrainian resistance, four civilians were killed in the town of Sievierodonetsk in a Russian bombardment, Luhansk governor Serhiy Haidai said.

Three other civilians were wounded in the attack on Wednesday, and the shelling continued into early Thursday, Mr Haidai said.

On the Russian side of the border, the governor of Kursk province said a truck driver was killed and several other civilians wounded by shelling from Ukraine.

Separatist authorities in the Donetsk region in eastern Ukraine said two civilians were killed and five wounded also in Ukrainian shelling over the last 24 hours.

Mariupol was a target of the Russians from the outset as Moscow sought to open a land corridor from its territory to the Crimean Peninsula, which it seized from Ukraine in 2014.

The city - its pre-war population of about 430,000 now reduced by about three-quarters - has largely been reduced to rubble by relentless bombardment, and Ukraine says over 20,000 civilians have been killed there.