KYIV (AFP) - Ukraine announced on Saturday (June 4) the deaths of four foreign military volunteers fighting Russian forces, whose invasion has spurred a wave of solidarity abroad including from experienced combat veterans.

The International Legion of Defence of Ukraine, an official volunteer brigade, announced the men from Germany, the Netherlands, Australia and France had died but did not specify when or under what circumstances.

"We lost our brothers in combat but their bravery, their memory and legacy will forever inspire us," it said in a statement.

AFP journalists in Kharkiv last month were present at the May 21 burial of the Dutch citizen named in the statement, Mr Ronald Vogelaar. Colleagues said he had been killed several days earlier by artillery.

The Australian was reported killed in May and his death was confirmed by a foreign affairs department spokesman.

Tasmania's Mercury newspaper identified the man as MR Michael Charles O'Neill, 47. A tribute on Facebook said he was "driving the wounded and injured from the front line".

Paris on Friday confirmed that a French volunteer fighter had been killed in combat in Ukraine, following reports that the man died in artillery fire in the Kharkiv region.

The official statement on Saturday was accompanied by photographs of the men showing them clad in camouflage and carrying small arms.

The organisation says on its website that citizens from countries including Denmark, Israel, Poland, Latvia, Croatia the United Kingdom, the Netherlands and Canada have joined their ranks.

Ukraine President Volodymyr Zelensky announced the formation of the force at the start of the war, and Kyiv shortly afterwards said it had received some 20,000 applications.

The conflict, now beyond its 100th day, is also said to have drawn in a significant number of volunteer fighters from ex-Soviet state Georgia, which fought a brief and disastrous war with Russia in 2008.

Russia claimed this week to have killed "hundreds" of foreign fighters in Ukraine since its invasion began on Feb 24 and stemmed the flow of newcomers.