The US Defense Department warned Americans on Friday against going to Ukraine to join the fight against Russia's invasion after a former marine was killed.

"We continue to urge Americans not to go to Ukraine... this is an active war zone, this is not the place to be traveling to," Pentagon spokesman John Kirby told CNN the day after reports emerged of the death of Willy Joseph Cancel.

Cancel's mother, Rebecca Cabrera, told CNN her 22-year-old son was killed on Monday in Ukraine while working with a private military contractor, having traveled there in mid-March. 

She said her son's body had not yet been recovered. He is survived by his wife and their child, who is not yet one-year-old.

His wife, Brittany Cancel, told ABC News that her husband "was eager to volunteer" to go to Ukraine and "went there wanting to help people."

Kirby expressed condolences for Cancel's family and said he understood his "altruistic motives," while underscoring that there are ways to support Ukraine "in a safe, effective way."

He added that the Pentagon did not have information on how Cancel was killed.

Early after Russia's invasion, launched on February 24, Ukrainian President Volodymyr Zelensky called for foreigners to volunteer to fight for his country.

In March, Ukraine's foreign ministry said tens of thousands of volunteers from dozens of countries had answered the call.

London on Thursday reported what was believed to be the first death of a Briton fighting in the conflict. Another Briton is also missing.