One of the two ringleaders of a far-right US militia group which plotted to kidnap and kill Michigan Governor Gretchen Whitmer was sentenced to 16 years in prison on Tuesday, the Justice Department announced.

Adam Fox, 39, was convicted alongside co-defendant Barry Croft, 47, in August for conspiracy in planning the Democratic governor's kidnapping and preparing to use weapons of mass destruction in the 2020 plot.

He and several others in the militia, including an FBI informant, planned the attack on Whitmer in part as a response to her Covid-19 quarantine policies but also, they had said, to start a civil war.

The plot involved kidnapping Whitmer from her vacation cottage near Elk Rapids, Michigan, and using improvised bombs targeting security officers to help in their escape, according to the Justice Department.

Fox faced a possible life sentence but was given 16 years in prison.

Croft is to be sentenced on Wednesday.

"Today's sentence reflects the Department of Justice's unwavering commitment to protecting our elected officials, law enforcement officers, and dedicated public servants from criminal threats and violence," said Assistant Attorney General Matthew Olsen in a statement.

The sentence came 12 days after three others who participated in the plot received sentences ranging from seven to 12 years for their part in a Michigan state court, where they had been charged with domestic terrorism. (AFP)