A 98-year-old Minnesota resident has been accused by Poland of being a Nazi war criminal.

Michael Karkoc, a naturalised US citizen, has been accused of being a Waffen SS soldier who commanded a unit of Ukrainian nationalists responsible for mass killings in villages along Poland’s eastern border during the war.

Poland has issued an arrest warrant for Mr Karkoc, who has denied the allegations through his son.

Kevin Jon Heller, an international criminal law professor at the University of London and University of Amsterdam, told The Independent that Mr Karkoc is “fine in Minnesota, for now.”

In order for Mr Karkoc to be transferred, Poland would have to file a formal request for extradition and “there is no question war crimes of this nature are extraditable offenses,” Mr Heller said.

However, Mr Heller said his son’s claim that Mr Karkoc suffers from Alzheimer’s “make[s] a huge difference.”

His age is not as much of a factor in extradition and prosecution, said Mr Heller.

Germany, which has prosecuted accused Nazi soldiers well into their 90s, previously made the allegation of Mr Karkoc, based on historical evidence.

But Berlin dropped the charges when it deemed him unfit to stand trial due to his health.

One other “wrinkle” in the case, as Mr Heller put it, is that Mr Karkoc allegedly lied on his immigration forms to enter and remain in the US since the late 1940s.

According to reporting done by the Associated Press in 2013, Mr Karkoc said he never served in the military during WWII.

Had he admitted to being an SS officer, he would not have been allowed into the US.

“If he did in fact lie, the US could strip his naturalisation and extradite him,” Mr Heller explained.

He noted that though the US is allowed to refuse extradition of Mr Karkoc to Poland, they have an obligation to prosecute him in the US. “It doesn’t mean they actually have to, though,” Mr Heller said.

Given the timing of Nazi war criminal prosecutions and the age of many of the accused, Mr Heller said “there is a much bigger historical context” to the case.

“The stain of these [Nazi] crimes never goes away,” Mr Heller said.