A former Australian soldier has been killed while fighting in Ukraine, authorities have confirmed.

The memorial page for the International Legion Defense of Ukraine posted that Victorian man Sage O’Donnell had died. 

“The Australian Sage O’Donnell, who had been serving in Ukraine as a Volunteer, succumbed on the Battlefield,” the post reads.

“Honor, Glory and Gratitude To Our Brother.”

The timing of Mr O’Donnell’s death remains unconfirmed, but is believed to have occurred in the lead up to Christmas.

In a post on social media, Mr O’Donnell’s father said his son had died fighting Russian President Vladimir Putin.

Mr O’Donnell’s LinkedIn profile says he served in the Australian Army for almost six years.

A Department of Foreign Affairs and Trade spokeswoman said it was providing “consular assistance to the family of an Australian who died in Ukraine”.

“Owing to our privacy obligations we are unable to comment further,” the spokeswoman said.

By Tess Ikonomou in Canberra