WASHINGTON, Jan. 12 (CNA) - United States Vice President Mike Pence said on Tuesday (Jan 12) that he is opposed to invoking the 25th Amendment to remove President Donald Trump from office, after Trump's supporters laid siege to the U.S. Capitol last week.

In a letter late Tuesday to House Speaker Nancy Pelosi, Pence said the mechanism should not be used “as a means of punishment or usurpation” and reserved for cases of medical or mental incapacitation.

Pelosi has called on Pence to secure the majority of the Cabinet and votes to declare Trump unfit to serve.

"With just eight days left in the President's term, you and the Democratic Caucus are demanding that the Cabinet and I invoke the 25th Amendment," Pence wrote, referring to the process that would declare Trump unable to fulfil his duties and install Pence as acting president for the remainder of the term.

"I do not believe that such a course of action is in the best interest of our nation or consistent with our Constitution," he said, hours before the House was to vote on a measure calling on him to initiate the 25th Amendment process or risk an impeachment vote against Trump.

At least three Republicans, including a member of the House leadership, on Tuesday said they would vote to impeach Trump for urging supporters to march on the Capitol and "fight" shortly before the attack that led to the deaths of five people.

Pence told Pelosi the administration's energy was focused on ensuring an orderly transition, and appealed to her and other members of Congress to avoid actions that "would further divide and inflame the passions of the moment".

"Work with us to lower the temperature and unite our country as we prepare to inaugurate President-elect Joe Biden as the next President of the United States," Pence said.

Pence and Trump had their first conversation at the White House on Monday evening after days of silence following the riot and Trump's public rebuke of Pence for not seeking to block the congressional certification of Biden's Electoral College win.

Trump, who has sought unsuccessfully to overturn his Nov 3 election loss, had pressured Pence to intervene in the certification process, and some of Trump's supporters in the assault discussed assassinating Pence for being a traitor.

Pence referenced the strains in his letter to Pelosi.

"Last week, I did not yield to pressure to exert power beyond my constitutional authority to determine the outcome of the election, and I will not now yield to efforts in the House of Representatives to play political games at a time so serious in the life of our nation," he wrote.

Removal of a president under the 25th Amendment requires a declaration by the vice president and the majority of the president's Cabinet.

Pelosi has said Pence's failure to trigger that process would lead to a Trump impeachment vote on Wednesday. The vote would make him the first American president to be impeached twice.

While the previous three impeachments - those of Presidents Andrew Johnson, Bill Clinton and Trump - took months before a final vote, including investigations and hearings, this time it will have only taken a week.

For now, the Republican-led Senate is not expected to hold a trial and vote on whether to convict Trump before Biden is sworn in Jan 20. Still, Democrats feel that action by the House would send an important message to the country.