Besides the 561 dead, an additional 1,697 troops had been wounded since the invasion began on February 24, National Guard chief Oleksiy Nadtochy said in an online briefing.

Wednesday's statement marked a rare move as both Ukrainian and Russian officials have been tight-lipped about their losses in the war.

Figures about troops killed in battle have very rarely been released by Ukrainian officials, with neither the defence ministry in Kyiv nor its counterpart in Moscow offering any information on their own military losses.

In mid-April, President Volodymyr Zelensky said between 2,500 and 3,000 Ukrainian soldiers had been killed while around 10,000 others had been wounded, admitting it was "difficult to say how many of them would survive".

Ukraine's National Guard, which falls under the interior ministry, was created in March 2014 as Russia seized control of the Black Sea peninsula of Crimea and massed troops on Ukraine's eastern border.

By law, it can have up to 60,000 soldiers in its ranks and has notably absorbed several self-defence groups that were on the frontline of the 2014 Maidan revolution, as well as various nationalist outfits like Azov.

Previously known as the "Azov Battalion", the unit was created in 2014 by far-right activists and first deployed against pro-Russian separatists in eastern Ukraine.

It has since shaken off the far-right ideology and been integrated into the National Guard, experts say. It is now known as the "Azov Regiment" and has a reputation for being a tough fighting unit.

Kyiv on Tuesday said more than 1,000 fighters remain trapped inside the besieged Azovstal steel plant in the southern port city of Mariupol, hundreds of whom are injured.

Some Azov soldiers have also died at the plant, but it remains unclear how many.