Thirteen men, including members of two right-wing militias, have been arrested for plotting to kidnap the Democratic governor of the US state of Michigan and spark a "civil war," officials said Thursday.

Michigan Governor Gretchen Whitmer thanked law enforcement for thwarting the kidnap plot and rebuked President Donald Trump for failing to condemn hate groups.

Announcing the arrests, Michigan Attorney General Dana Nessel described the conspiracy to kidnap the 49-year-old Whitmer as a "serious and credible threat."

Andrew Birge, US attorney for the western district of Michigan, said six men were facing federal charges for conspiring to kidnap Whitmer from her vacation home.

Birge said they had conducted surveillance of her residence and had even tested an improvised explosive device they intended to use to divert law enforcement away from her home.

They were arrested by FBI agents and Michigan police on Wednesday as they met to "pool funds for explosives and exchange tactical gear," the Justice Department said.

Whitmer has been attacked repeatedly by Trump this year over her tough coronavirus lockdown and armed right-wing groups have staged protests in the state capital Lansing against her stay-at-home orders.

"LIBERATE MICHIGAN!" Trump tweeted in April.

"When I took the oath of office 22 months ago, I knew this job would be hard," said Whitmer, who was on the shortlist to become Democrat Joe Biden's vice presidential candidate.

"I'll be honest, I never could have imagined anything like this," she said.

"None of us have faced a challenge like Covid," Whitmer added. "We are not one another's enemy. This virus is our enemy."

As for Trump, Whitmer pointed to his refusal at the recent debate with Biden to condemn white supremacists and "hate groups" like the two Michigan militias that allegedly targeted her.

"Stand back and stand by, he told them," she said.

"Hate groups heard the president's words not as a rebuke but as a rallying cry, as a call to action," she added. "When our leaders speak, their words matter. They carry weight.

"When our leaders meet with, encourage, or fraternize with domestic terrorists they legitimize their actions, they are complicit."

Trump's November opponent, Biden, also sought to pin some blame on the Republican president.

"The words he utters matter," Biden said at a campaign event. "Why won't the president just say 'stop. Stop, stop, stop.' And we will pursue you if you don't."

- 'Violent overthrow' -

Nessel, the attorney general, said that in addition to the six men arrested for plotting to kidnap Whitmer, seven members of a militia group called the Wolverine Watchmen were facing state charges.

They are suspected of attempting to identify the home addresses of law enforcement officers in order to target them to "instigate a civil war," Nessel said.

They also planned to attack the capitol building in Lansing and kidnap government officials including Whitmer, she said.

The seven were charged with various offenses including providing material support for terrorist acts, gang membership and firearms violations.

A sworn affadavit from an FBI agent detailed the outlines of the plot to kidnap Whitmer.

The FBI said it became aware through social media in early 2020 "that a group of individuals were discussing the violent overthrow of certain government and law-enforcement components."

A confidential informant attended a meeting in June of around 14 people, the FBI said, where the group "talked about creating a society that followed the US Bill of Rights and where they could be self-sufficient."

"They discussed different ways of achieving this goal from peaceful endeavors to violent actions," the affadavit said.

"At one point, several members talked about state governments they believed were violating the US Constitution, including the government of Michigan and Governor Gretchen Whitmer," it said.

"Several members talked about murdering 'tyrants' or 'taking' a sitting governor," it said.

The six men charged with conspiring to kidnap Whitmer were identified as Adam Fox, Barry Croft, Ty Garbin, Kaleb Franks, Daniel Harris, and Brandon Caserta.

Croft is a resident of Delaware while the other five live in Michigan.

The affadavit said Fox was recorded in June saying he needed 200 men to "storm the Capitol building in Lansing, Michigan, and take hostages, including the governor."

"Fox explained they would try the governor of Michigan for 'treason,' and he said they would execute the plan before the November 2020 elections," the affadavit said.