House Democrats have asked President Donald Trump's former national security advisor John Bolton to testify in their impeachment probe on Nov. 7, NBC News reported Wednesday.

The top lawyer for the National Security Council, John Eisenberg, has been called testify next Monday, as has Michael Ellis, another lawyer for that council, which advises presidents on security and foreign policy issues, NBC reported.

Bolton's lawyer, Charles Cooper, responded to the House request for his testimony, telling NBC News: "Bolton is not willing to appear voluntarily."

Cooper added: "I stand ready at all times to accept service of a subpoena on his behalf."

The White House also did not immediately respond to a request for comment.

It was not immediately clear if Eisenberg or Ellis would show up for their requested depositions — or if the White House would seek to block their appearances.

Last Friday, NBC News reported that lawyers for Bolton were in touch with House officials about his possible testimony.

On that same day, the former deputy national security advisor Charles Kupperman asked a federal judge to rule on whether he can be forced to testify to the House, which had issued a subpoena to him. The White House has moved to block his deposition by invoking a claim of so-called constitutional immunity.

Kupperman's lawyer in that case, Cooper, also represents Bolton.

The House is investigating Trump's pressuring of Ukraine to launch an investigation of former Vice President Joe Biden.

At the time that Trump asked Ukraine President Volodymyr Zelensky for that inquiry into Biden, who is seeking the 2020 Democratic presidential nomination, the White House was withholding nearly $400 million in military aid from Ukraine.

Trump also requested that Ukraine investigate a conspiracy theory that that country played a role in framing Russian agents for hacking of Democratic Party computers in the run-up to the 2016 presidential election.