KYIV: 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 underway over the fate of the last besieged defenders in the southern port city of Mariupol.

Europe on Friday pledged another half a billion dollars in military support for Ukraine as it resists the Russian invasion that began on February 24, while Sweden's and Finland's moves to apply for North Atlantic Treaty Organization (NATO) membership hit some obstacles.

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

"There's heavy fighting on the border with [the] Donetsk region, from the side of Popasna," he reported, adding that the Russians suffered significant equipment and personnel losses.

"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 armored vehicles on a riverbank and wrecked pontoon bridges.

The United Kingdom's Ministry of Defense said the Russians sustained heavy losses after Ukrainian forces prevented their attempted river crossing.

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

Russian forces have "failed to make any significant advance despite concentrating forces in this area," it noted.

A senior United States defense official said most of the military activity was now in the Donbas "and essentially, we continue to see the Russians not making any major gain" there.

The latest assessment from the US-based Institute for the Study of War said "Russian President Vladimir Putin likely intends to annex occupied southern and eastern Ukraine the coming months."

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

'Difficult negotiations'

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

"Of course, we remember every city and every community that is still under occupation," he said, with a special mention of Mariupol, where the last defenders are holed up in the vast Azovstal steel plant.

"We do not stop trying to save all our people from Mariupol and Azovstal," the leader added.

"Currently, very difficult negotiations are underway on the next stage of the evacuation mission: the rescue of the seriously wounded [and] medics. It is a large number of people," he said.

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

The Ukrainian General Staff said in an update on Saturday that Russian forces continued "to blockade our units near the Azovstal plant" and carried out "massive artillery and airstrikes."

From inside the plant, Sviatoslav Palamar, one of the leaders of the Ukrainian Azov regiment, told the online Kyiv Security Forum that there were 600 wounded there and pleaded for help to evacuate them.

"We continue to defend ourselves, and we shall not surrender," he said.

NATO bids

At a meeting of the Group of Seven (G7) foreign ministers in Germany on Friday, European Union foreign policy chief Josep Borrell promised Ukraine an extra 500 million euros ($520 million), bringing the bloc's total military aid to 2 billion euros.

The meeting of ministers representing seven of the world's wealthiest nations, joined by their Ukrainian and Moldovan counterparts, will continue on Saturday.

An informal meeting of NATO foreign ministers is also scheduled in Berlin on Saturday, where the Swedish and Finnish ministers plan to meet their Turkish counterpart to discuss their potential bids to join the military alliance.

That follows Turkish President Recep Tayyip Erdogan expressed opposition to their entry.