The US House of Representatives' Foreign Affairs Committee has launched a probe into President Donald Trump's decision to fire a government watchdog reportedly investigating Secretary of State Mike Pompeo.

The Democratic-led committee said it would launch the investigation along with colleagues on the Senate Foreign Affairs Committee, which is controlled by Mr Trump's fellow Republicans.

Mr Trump notified House of Representatives speaker Nancy Pelosi late yesterday that he planned to dismiss State Department Inspector General Steve Linick.

It was his third abrupt dismissal of an official tasked with monitoring governmental wrongdoing since April.

Democrats denounced it as part of what they said was a troubling pattern of the president undermining the traditionally independent watchdogs.

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.

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, Susan Pompeo.

CNN, citing a senior State Department official, reported that it was Mr Pompeo himself who had recommended the firing and who hand-picked Stephen Akard, a former aide to Vice President Mike Pence, to succeed Mr Linick. 

By law, the administration must give Congress 30 days' notice of its plans to formally terminate an inspector general, in theory giving lawmakers time to study and potentially protest the move.

However previous such firings have gone through unimpeded, and inspectors general previously dismissed have been replaced by political allies of the president.

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

CNN reported last year that a whistleblower had complained that the Bureau of 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 takeout food or tending to the family dog.

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 also confirmed that the new inspector general would be Mr Akard, an attorney who served as a foreign affairs advisor to Mr Pence when the latter was governor 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.

Ms 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," she 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, 56, 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 Linick, a long-time 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 Trump removed as the US ambassador to Ukraine.