House Speaker Nancy Pelosi spoke to the top US military commander on Friday about taking precautions to ensure that Republican President Donald Trump cannot initiate hostilities or order a nuclear strike in his remaining 12 days in office.

Pelosi said in a letter to Democratic lawmakers that she spoke to Army General Mark Milley, chairman of the Joint Chiefs of Staff, about what measures are available to rein in the Republican president.

Trump, angry about his election loss, incited supporters in the days before an invasion of the US Capitol on Wednesday.

“The situation of this unhinged president could not be more dangerous, and we must do everything that we can to protect the American people from his unbalanced assault on our country and our democracy,” Pelosi said in a letter to colleagues.

While Pelosi's letter highlights the concern among lawmakers about what Trump may try to do during his remaining time in office, there are questions about what Milley or anyone could actually do to stop a president from using nuclear weapons.

“There is no legal way to do this. The president has sole, unfettered authority to order the use of nuclear weapons with no 'second vote' required,” said Jeffrey Lewis, a professor at the Middlebury Institute of International Studies at Monterey in California.

Milley's office said that Pelosi had initiated the call and Milley “answered her questions regarding the process of nuclear command authority”. A US official, speaking on condition of anonymity, said that any use of nuclear weapons is a highly deliberative process.

Democratic President-elect Joe Biden takes office on January 20. Trump said earlier on Friday he would not attend the inauguration, breaking with long-standing tradition in American presidential transitions.

Pelosi, the top Democrat in Congress, also said she had not heard back from Vice President Mike Pence about whether he would agree to top Democrats' request that he invoke the 25th Amendment to remove Trump from office “for his incitement of insurrection and the danger he still poses”.

She said “we still hope to hear from him as soon as possible with a positive answer.”

Pelosi and Senator Chuck Schumer, the top Democrat in the Senate, have said that if Pence does not agree to the request, Democrats were prepared to impeach Trump a second time.