Members of President Donald Trump's cabinet on Wednesday discussed the possibility of removing Donald Trump from office after his supporters stormed the Capitol, three US news channels reported.

The discussions focused on the 25th amendment to the US Constitution, which allows for a president's removal by the vice-president and cabinet if he is judged "unable to discharge the powers and duties of his office." 

Invoking it would require Vice-President Michael Pence to lead the cabinet in a vote on removing him.

CNN quoted unnamed Republican leaders saying the 25th amendment had been discussed, saying they had described Trump as "out of control."

CBS reporter Margaret Brennan said that "nothing formal" had been presented to Pence, and ABC reporter Katherine Faulders said "multiple" sources had told her that discussions took place on the unprecedented move.

Trump's encouragement of the protesters, his unfounded claims that he lost the November 3 presidential election due to massive fraud, and other bizarre behavior have raised questions about his ability to lead.

While only two weeks remain before President-elect Joe Biden takes office, after the attacks on Congress Wednesday Democratic lawmakers called for invoking the 25th Amendment as well.

Democrats of the House Judiciary Committee sent a letter to Pence urging him to act to remove Trump, saying he had stoked an act of insurrection and "sought to undermine our democracy."

Pointing to a rambling speech Trump gave Wednesday, it said he "revealed that he is not mentally sound and is still unable to process and accept the results of the 2020 election."

Others blamed Trump for fueling terrorism.

"The President incited a domestic terror attack on the Capitol. He is an imminent threat to our democracy and he needs to be removed from office immediately," said Representative Kathleen Rice in a tweet.

"The Cabinet must invoke the 25th Amendment," she wrote.

The lawmakers' call was echoed by the influential Washington Post.

"Responsibility for this act of sedition lies squarely with the president, who has shown that his continued tenure in office poses a grave threat to US democracy. He should be removed," the Post said.

"The president is unfit to remain in office for the next 14 days. Every second he retains the vast powers of the presidency is a threat to public order and national security," they said.

Meanwhile, every living former US president on Wednesday denounced the violence of a mob that stormed the Capitol building in Washington, forcing lawmakers to flee to safety and leaving one woman dead.

The crowd, made up of supporters of President Donald Trump, opposed the certification underway in Congress of Joe Biden's presidential election win in November. 

George W. Bush called out fellow Republicans for fueling the "insurrection," likening the situation to a "banana republic."

"I am appalled by the reckless behavior of some political leaders since the election and by the lack of respect shown today for our institutions, our traditions and our law enforcement," Bush's statement said, in a barely veiled swipe at Trump. 

Barack Obama also blamed Republicans and Trump, "who has continued to baselessly lie about the outcome of a lawful election," he said in a statement. 

Trump's most recent predecessor called the incident "a moment of great dishonor and shame for our nation." 

"But we'd be kidding ourselves if we treated it as a total surprise," Obama said, calling the events of the day "the consequences" of Trump and his supporters refusing to accept the results of last year's election. 

Bill Clinton denounced the riot as "unprecedented assault" on the US Capitol and the nation itself.

"Today we faced an unprecedented assault on our Capitol, our Constitution, and our country," the Democratic former president said in a statement.

"The match was lit by Donald Trump and his most ardent enablers, including many in Congress, to overturn the results of an election he lost."

And the oldest member of the exclusive club, 96-year-old Democrat Jimmy Carter, said he was "troubled" by Wednesday's scenes, which he called a "national tragedy." 

"We join our fellow citizens in praying for a peaceful resolution so our nation can heal and complete the transfer of power as we have for more than two centuries," he said in a statement.