US House Speaker Nancy Pelosi said Friday she spoke with the nation's top military leader about ensuring that an "unhinged" President Donald Trump does not launch a nuclear attack in his final days in office.

The top Democrat also warned that Congress would swiftly take action to impeach the president if he does not resign, although the effort is receiving push back from Republican leadership.

Pelosi said she talked with Joint Chiefs Chairman Mark Milley "to discuss available precautions for preventing an unstable president from initiating military hostilities or accessing the launch codes and ordering a nuclear strike."

The extraordinary acknowledgement that Pelosi has spoken with military brass about restricting the president's powers came in a letter to her Democratic colleagues, and highlighted tensions over Trump's remaining days in the White House.

Democrats and some Republicans have accused Trump of fomenting an insurrection when a pro-Trump mob stormed and ransacked 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 wrote.

Military spokesman Colonel Dave Butler said Pelosi did call the joint chiefs chairman but would not provide specifics other than to say Milley "answered her questions regarding the process of nuclear command authority."

Pelosi in her letter also said she was prepared to launch impeachment proceedings against Trump if he does not voluntarily resign or if Vice President Mike Pence does not begin a process laid out in the 25th Amendment that allows him and the cabinet to remove the president.

"If the president does not leave office imminently and willingly, the Congress will proceed with our action," she said.

'Incitement of insurrection'?

Articles of impeachment have been drafted but they have yet to be introduced.

Support is growing among Democrats on Capitol Hill for ousting Trump after his supporters stormed the US Capitol seeking to prevent lawmakers from certifying Joe Biden's November election win.

Democratic and Republican lawmakers alike described the mayhem, in which five people died including a US Capitol Police officer, as an insurrection.

"There is growing momentum around the invocation of the 25th Amendment, which would allow the vice president and a majority of the cabinet to remove the president for his incitement of insurrection and the danger he still poses," Pelosi wrote.

Democrat Jackie Speier, a member of the House Intelligence Committee, said "all options" remain on the table.

"There is a group of us that feel very strongly that we need to move forward with the impeachment article for insurrection by the president," she told CNN.

"If we are not willing to state that the acts by the president of the United States to incite domestic terrorism and insurrection is an impeachable offense, then nothing is an impeachable offense."

But House Minority Leader Kevin McCarthy, the chamber's top Republican, pushed back.

"Impeaching the president with just 12 days left in his term will only divide our country more," he said.