Former US President Donald Trump will not testify under oath voluntarily for the Senate impeachment trial set to start next week, Trump's legal team has said, calling the House Democrats' request for Trump testifying a "stunt".

"We are in receipt of your latest public relations stunt," Trump's attorneys Bruce Castor and David Schoen wrote in a response on Thursday, Xinhua news agency reported.

"As you certainly know, there is no such thing as a negative interference in this unconstitutional proceeding. Your letter only confirms what is known to everyone: you cannot prove your allegations against the 45th President of the United States, who is now a private citizen," they continued.

Schoen later clarified by text message that Trump did not plan to testify voluntarily, according to a New York Times report.

"I don't think anyone being impeached would show up at the proceedings we firmly believe are unconstitutional," Schoen said.

The House managers could still attempt to subpoena testimony from Trump during the trial, but doing so would require support from a majority of the Senate, said the NYT report.

Earlier on Thursday, House Democrats asked Trump to testify under oath as early as next Monday and no later than next Thursday.

Lead Impeachment Manager Jamie Raskin, a former constitutional law professor, wrote in a letter that Trump's response to the article of impeachment had denied what managers called "incontrovertible facts about the president's conduct on and leading up to" the January 6 Capitol riots leaving five people dead.

"In light of your disputing these factual allegations, I write to invite you to provide testimony under oath, either before or during the Senate impeachment trial, concerning your conduct on January 6, 2021," Raskin wrote to Trump.

"If you decline this invitation, we reserve any and all rights, including the right to establish at trial that your refusal to testify supports a strong adverse inference regarding your actions (and inaction) on January 6, 2021," Raskin wrote.

In the wake of the Capitol riots, the Democrats-controlled House voted to impeach Trump over "incitement of insurrection" in a 232-197 vote on January 13, making him the first US president to be impeached twice.

The Senate impeachment trial is expected to begin on February 9.