Donald Trump fires State Department watchdog said to be investigating one of his top aides

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

Mr 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.

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

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A Democratic congressional aide, speaking on condition of anonymity, said Mr Linick was probing complaints that Mr 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 Mr Linick's firing but did not comment on the reason or on whether Mr Pompeo was under investigation.

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.

Mr 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 Mr 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," Ms Pelosi said.

Mr Pompeo is one of Mr Trump's most trusted aides - and a rare one never to come publicly into the crosshairs of the mercurial president.

In recent months Mr Pompeo has moved US foreign policy forcefully to the right - encouraging a drone strike that killed a top Iranian general and promoting a theory, discounted by mainstream scientists, that the COVID-19 pandemic originated in a Chinese laboratory.

Mr Pompeo, 56, is widely seen as entertaining presidential ambitions of his own. But he has frustrated some Republican colleagues who want him to quit and return to his state of Kansas to run in November for a Senate seat that is looking surprisingly vulnerable for the party.

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Mr Linick, a longtime prosecutor, was appointed in 2013 by Mr Trump's predecessor Barack Obama to oversee the $70 billion juggernaut of US diplomacy.

He played a small role in Mr Trump's impeachment saga last year, handing to Congress documents by Mr Trump's lawyer Rudy Giuliani with unproven claims about Democratic presidential candidate Joe Biden and Marie Yovanovitch, whom Mr Trump removed as the US ambassador to Ukraine.

Mr Trump repeated the charges to Ukraine's president and pressed him to dig up dirt, with his administration freezing military aid to the ally which is battling Russian-backed separatists.

Since his acquittal by the Senate, Mr Trump has fumed against a "Deep State" he sees as out to get him.

He has removed or demoted inspectors general for the Pentagon, the intelligence community and the Department of Health and Human Services, as well as a senior health official who questioned Mr Trump's promotion of unproven drug therapies for COVID-19.