U.S. President Joe Biden on Thursday threw his full support behind Sweden and Finland's bids to join NATO as he and European leaders said they were confident Turkey's surprise opposition to the Nordic states' membership could be addressed.

Meanwhile in Germany, Group of Seven financial leaders looked set to agree on Thursday and Friday on around $15 billion to help Ukraine pay its bills in coming months.

Finland and Sweden say they were spurred into joining NATO by Russia's invasion of Ukraine, reversing generations of military non-alignment in the biggest shake-up in European security for decades.

Turkey however has objected to the move, accusing the two Nordic states of harbouring Kurdish militants.

Biden hosted Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinist at the White House, a chance for Washington to demonstrate that Russia's invasion has backfired, bringing about the very expansion of NATO that Moscow has said it was fighting to halt.

"Finland and Sweden make NATO stronger," Biden said.

Asked if Turkey's concerns can be addressed, Biden told reporters: "I think we're going to be OK".

Turkey's President Tayyip Erdogan said late on Wednesday, "We have told allies that we will say no to Finland and Sweden's NATO membership," adding, "NATO is a security alliance and we cannot accept terrorists to be in it."

Niinisto said Finland would commit to Turkey's security, adding, "We condemn terrorism in all its forms and we are actively engaged in combating it."

The past week has seen Russia secure its biggest victory since the invasion began, with Kyiv announcing it had ordered its garrison in a steelworks in Mariupol to stand down, after a nearly three-month siege of the city.

The ultimate outcome of the bloodiest battle in Europe for decades has remained unclear, with no confirmation of the fate of hundreds of Ukrainian defenders. Moscow said on Thursday that 1,730 Ukrainian fighters had surrendered so far, including 771 in the past 24 hours.

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Ukraine, which says it aims to secure a prisoner swap, has not said how many were inside the plant or commented 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 Switzerland-based International Committee of the Red Cross said it has registered hundreds of prisoners from the plant now held by Russia, but it has not given a precise number.

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 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 were given medical treatment while those who were fit were 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 while negotiations are under way to rescue them.

Russia denies 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 ADVANCES

Russian forces have lost ground elsewhere, however, driven from northern Ukraine and the area around the capital at the end of March, and pushed this month from the outskirts of the second-largest city Kharkiv.

Oleksiy Gromov, Deputy Chief of the Main Operational Department of the General Staff of Ukraine's Armed Forces, told an online briefing that Ukraine had recaptured 23 settlements near Kharkiv in the last two weeks.

On Thursday, the crash of artillery duels resounded across sunlit fields and woodlands north of Kharkiv near the village Slatyne.

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

As life in the capital returns to normal, the United States reopened its embassy.

"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.

Serhiy Gaidai, governor of neighbouring Luhansk region, said four people had been killed in shelling of the frontline city of Sievierodonetsk.