The United States is withdrawing some personnel from bases in the Middle East, a US official said on Wednesday, after a senior Iranian official said Tehran had warned neighbours it would hit American bases if Washington strikes.

With Iran's leadership trying to put down the worst domestic unrest the Islamic Republic has ever faced, Tehran is seeking to deter U.S. President Donald Trump's repeated threats to intervene on behalf of anti-government protesters.

A US official, speaking on condition of anonymity, said the United States was withdrawing some personnel from key bases in the region as a precaution given heightened regional tensions.

Two European officials said US military intervention appeared likely, with one saying it could come in the next 24 hours. An Israeli official also said it appeared Trump had taken a decision to intervene, though the scope and timing had yet to be made clear.

Qatar said drawdowns from its Al Udeid Air Base, the biggest US base in the region, were "being undertaken in response to the current regional tensions".

Three diplomats said some personnel had been told to leave the base, though there were no immediate signs of large numbers of troops being bussed out to a soccer stadium and shopping mall as took place hours before an Iranian missile strike last year.

Trump has repeatedly threatened to intervene in support of protesters in Iran, where thousands of people have been reported killed in a crackdown on the protests against clerical rule.

Iran and its Western foes have both described the unrest, which began two weeks ago as demonstrations against dire economic conditions and rapidly escalated in recent days, as the most violent since the 1979 Islamic Revolution that installed Iran's system of clerical rule.

An Iranian official has said more than 2,000 people have died. A rights group put the toll at more than 2,600.

Iran "had never faced this volume of destruction", Armed Forces Chief of Staff Abdolrahim Mousavi said on Wednesday, blaming foreign enemies. French Foreign Minister Jean-Noel Barrot described "the most violent repression in Iran's contemporary history".

Iranian authorities have accused the United States and Israel of fomenting the unrest, carried out by people it calls armed terrorists.