Former National Security Adviser John Bolton has been invited by Democrats to appear for a deposition on Capitol Hill as part of their impeachment inquiry, a source familiar with the situation told Fox News.

Deposition notices have also gone out to John Eisenberg, a deputy assistant to President Trump, and Michael Ellis from White House counsel's office. Both Eisenberg and Ellis have served as legal advisers to the National Security Council, the source said.

Bolton has retained legal counsel. Attorney Charles Cooper said his client won't appear without a subpoena, which Democrats could issue.

Should he appear, Bolton will be the highest-profile official to be interviewed so far.

JOHN BOLTON RETAINS COUNSEL AMID SPECULATION OVER POSSIBLE APPEARANCE IN IMPEACHMENT INQUIRY

Bolton departed the White House last month over disagreements with the president on a range of foreign policy issues, including a planned troop drawdown in Afghanistan.

Last week, Bill Taylor, the top American diplomat to Ukraine, testified that he was told Bolton was concerned while serving in the White House about Trump’s actions toward Ukraine.

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Taylor said he was told Bolton referred to this as a “drug deal."

Earlier this month, Fiona Hill, the former senior director for Russian and Eurasian affairs, told lawmakers that a July meeting with Ukrainian and U.S. officials about desired investigations left her and Bolton so concerned that he directed her to alert a lawyer in the National Security Council.

Fox News’ Mike Emanuel, Chad Pergram and The Associated Press contributed to this report.