US State Department inspector general Steve Linick leaves after holding a briefing with lawmakers in Washington. File/AFP

A government watchdog fired suddenly by Donald Trump was investigating Secretary of State Mike Pompeo, said a Democratic lawmaker who accused the president of committing a potentially illegal act of retribution.

Trump sacked State Department Inspector General Steve Linick late on Friday in his latest abrupt dismissal of an official tasked with monitoring wrongdoing inside a government agency.

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Eliot Engel, who heads the House Foreign Affairs Committee, said he had learned that Linick had opened an investigation into Pompeo.

"Mr. Linick's firing amid such a probe strongly suggests that this is an unlawful act of retaliation," the congressman said in a statement.

US President Donald Trump poses for the media photographers. File photo

A Democratic congressional aide, speaking on condition of anonymity, said Linick was probing complaints that Pompeo misused a political appointee to perform personal tasks for himself and his wife.

Washington's top diplomat has raised eyebrows for frequently traveling the world on his government plane with his wife Susan Pompeo, who has no official role.

CNN reported last year that a whistleblower had complained that Diplomatic Security, which guards US missions overseas as well as the secretary of state, had been assigned questionable tasks for the Pompeos such as picking up the family dog and takeout food.

The State Department confirmed Linick's firing but did not comment on the reason or on whether Pompeo was under investigation.

Secretary of State Mike Pompeo attends a meeting. File photo

A State Department spokesperson said the new inspector general will be Stephen Akard, a former aide to Vice President Mike Pence from his home state of Indiana.

Akard since last year has led the State Department's Office of Foreign Missions, which handles relations with diplomats in the United States.

House Speaker Nancy Pelosi said Linick was "punished for honorably performing his duty to protect the Constitution and our national security."

"The president must cease his pattern of reprisal and retaliation against the public servants who are working to keep Americans safe, particularly during this time of global emergency," Pelosi said.