KYIV (AFP, REUTERS) - Intense fighting raged in eastern Ukraine's Donbas region where Russia has been concentrating its forces without making significant progress, while "very difficult negotiations" were under way over the fate of the last besieged defenders in the city of Mariupol.

Europe on Friday (May 13) pledged another €500,000 (S$725 million) in military support for Ukraine as it resists the Russian invasion that began on Feb 24.

At the end of March, after failing to take the capital Kyiv in the face of determined resistance, Russia turned its focus to eastern Ukraine.

Ukrainian forces have repulsed Russian attempts to cross a river and encircle the city of Severodonetsk, said Mr Serhiy Gaidai, governor of the eastern Luhansk region.

"There is heavy fighting on the border with Donetsk region, from the side of Popasna," he said, reporting heavy losses of equipment and personnel by the Russians.

"From interceptions (phone calls), we understand that a whole (Russian) battalion has refused to attack because they see what's happening."

Aerial images showed dozens of destroyed armoured vehicles on the river bank and wrecked pontoon bridges.

Britain's Ministry of Defence said the Russians had sustained heavy losses after Ukrainian forces successfully prevented their attempted river crossing.

"Conducting river crossings in a contested environment is a highly risky manoeuvre and speaks to the pressure the Russian commanders are under to make progress in their operations in eastern Ukraine," it said.

Russia's hopes of swift gains appear to have been thwarted and Ukraine has even managed to push Russian troops out of the northern city of Kharkiv, which had been a priority target for Moscow.

In Kharkiv region, "the enemy's main efforts are focused on ensuring the withdrawal of its units from the city of Kharkiv", said the spokesman for the Ukrainian General Staff of the Armed Forces.

"The gradual liberation of the Kharkiv region proves that we will not leave anyone to the enemy," Ukrainian President Volodymyr Zelensky said in his daily video address on Friday.

"We do not stop trying to save all our people from Mariupol and Azovstal," he said. "Currently, very difficult negotiations are under way on the next stage of the evacuation mission - the rescue of the seriously wounded, medics. It is a large number of people."

Women, children and the elderly who had taken refuge in the tunnels and bunkers in the Azovstal plant were evacuated at the end of last month with the help of the United Nations and Red Cross.

Ukraine has proposed evacuating 38 of the most severely wounded defenders, offering to release a number of Russian prisoners of war in return.

Nato member Turkey on Saturday has proposed carrying out a sea evacuation of wounded fighters holed up in a steel works in the southern Ukrainian city of Mariupol, President Recep Tayyip Erdogan’s spokesman said.

Mr Ibrahim Kalin told Reuters in an interview that he had personally discussed the proposal with Mr Zelensky in Kyiv two weeks ago and that it remains “on the table”, although Moscow has not agreed to it.

Under the plan, people evacuated from the vast Azovstal steel plant would be taken by land to the port of Berdyansk, which like Mariupol is on the Sea of Azov, and a Turkish vessel would take them across the Black Sea to Istanbul, he said.

“If it can be done that way, we are happy to do it. We are ready. In fact our ship is ready to go and bring the injured soldiers and other civilians to Turkey,” said Mr Kalin, who is also Erdogan’s top foreign policy adviser.

At a meeting in Germany of foreign ministers from the Group of Seven nations, European Union foreign policy chief Josep Borrell on Friday promised Ukraine an extra €500 million, bringing the bloc's total military aid to €2 billion.

The United States Senate's minority leader Mitch McConnell on Saturday led a delegation of Republican senators to Kyiv, where they met Mr Zelensky in a previously unannounced visit.

The Republicans' visit comes two weeks after Speaker Nancy Pelosi led a group of House Democrats to Kyiv.

Meanwhile, the bodies of Russian soldiers killed in Ukraine were brought to a rail yard outside Kyiv and stacked with hundreds of others in a refrigerated train, waiting for the time when they can be sent back to their families.

"Most of them were brought from the Kyiv region; there are some from Chernihiv region and from some other regions too," Mr Volodymyr Lyamzin, the chief civil-military liaison officer, told Reuters on Friday as stretcher-bearers in white, head-to-toe protective suits lifted body bags into the box cars.

He said refrigerated trains stationed in other regions across Ukraine were being used for the same grim purpose.

Mr Putin's most tangible success in Ukraine has been to capture a swathe of territory along the southern coast to link the Crimean peninsula - which Russia seized in 2014 - with the south-eastern regions of Donetsk and Luhansk, where Russian separatists have fought Ukrainian forces for years.