Ukraine fighters surrender in Mariupol

Ukraine confirmed the surrender of more than 250 fighters from the Azovstal steelworks. -EPA

Moscow says 1730 Ukrainian fighters have surrendered in Mariupol over three days, including 771 in the past 24 hours, claiming a surrender on a far bigger scale than Kyiv has acknowledged since ordering its garrison to stand down.

The ultimate outcome of Europe's bloodiest battle for decades remained publicly unresolved, with no confirmation of the fate of the hundreds of Ukrainian troops who had held out in a vast steelworks at the end of a near three-month siege.

Ukraine, which says it aims to secure a prisoner swap, has declined to say how many were inside the plant or comment on the fate of the rest, since confirming that just over 250 had surrendered in the initial hours after it ordered them to yield.

The leader of Russian-backed separatists in control of the area said nearly half of the fighters remained inside the steelworks, where underground bunkers and tunnels had protected them from weeks of Russian bombardment.

"More than a half have already left - more than half have laid down their arms," Denis Pushilin told the Solovyov Live internet television channel. "Let them surrender, let them live, let them honestly face the charges for all their crimes."

The wounded had been given medical treatment while those who were fit had been taken to a penal colony and were being treated well, he said.

Ukrainian officials say they cannot comment publicly on the fate of the fighters, as negotiations are under way behind the scenes to rescue them.

"The state is making utmost efforts to carry out the rescue of our service personnel," military spokesman Oleksandr Motuzaynik told a news conference. "Any information to the public could endanger that process."

Russia denies that it agreed to a prisoner swap. Many of the Azovstal defenders belong to a Ukrainian unit with far-right origins, the Azov Regiment, which Moscow calls Nazis and says must be prosecuted for crimes. Ukraine calls them national heroes.

The end of fighting in Mariupol, the biggest city Russia has captured so far, allows Russian President Vladimir Putin to claim a rare victory in the invasion he launched on February 24. It gives Russia complete control of the Sea of Azov and an unbroken stretch of territory along eastern and southern Ukraine.

Ukraine says tens of thousands of civilians died in nearly three months of Russian bombardment that laid the city to waste. 

Moscow denies targeting civilians in its "special military operation" to disarm and "de-Nazify" its neighbour. 

Russian forces were driven from northern Ukraine and the area around the capital at the end of March, and have been pushed this month from the outskirts of the second-largest city Kharkiv by a Ukrainian counter-offensive.

Ukrainian troops pressing on with their advance said fighting was under way around the nearby village of Demetiivka, which the Ukrainian military said was recaptured the previous day, only about 8km from the Russian border.

In a sign of the return of normal life in the capital, the United States reopened its embassy on Wednesday.

"The Ukrainian people... have defended their homeland in the face of Russia's unconscionable invasion, and, as a result, the Stars and Stripes are flying over the Embassy once again," said Secretary of State Antony Blinken.

But Russia is still pressing its main offensive using massed artillery and armour, trying to capture more territory in the eastern Donbas, comprised of the Donetsk and Luhansk regions, which Moscow claims on behalf of separatists.

Ukraine's general staff said Russia's attacks were focused on Donetsk. Russian forces "suffered significant losses" around Slovyansk to the north of Donetsk.

Police said on Thursday that two children had been killed in the Donetsk city of Lyman. Serhiy Gaidai, governor of neighbouring Luhansk region, said four people had been killed and three wounded in shelling of the frontline city of Sievierodonetsk.

In Russia, the regional governor of the Kursk border region accused Ukrainian forces of shelling a border village, killing at least one civilian.