US President Donald Trump fired State Department Inspector General Steve Linick, saying he had lost confidence in the watchdog's ability to serve.

Trump did not give a reason for his loss of confidence but said in a letter late Friday that his removal would be effective in 30 days.

Linick, who was appointed inspector general in 2013 during the Obama administration, is the latest in a series of independent government watchdogs to be ousted in recent weeks.

Democratic lawmakers heavily criticized Linick's removal, suggesting he may have been targeted for opening an investigation into Secretary of State Mike Pompeo.

Read more: US drops criminal case against ex-Trump adviser Michael Flynn

'Act of retaliation'

House Foreign Affairs Committee Chairman Eliot Engel accused Trump of acting to protect "one of his most loyal supporters."

"Mr Linick's dismissal in the midst of such an investigation strongly suggests that this is an unlawful act of retaliation," he said.

He did not elaborate on the details of the alleged probe, but a Democratic congressional aide cited by news agency Agence France-Presse said Linick was looking into claims that Pompeo had misused a political appointee to carry out personal tasks for himself and his wife.

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

She said the dismissal marked an acceleration of "a dangerous patter of retaliation" from Trump.

Pelosi should shut up and go suck some d****k