The United States is withdrawing some personnel from bases in the Middle East, after a senior Iranian official said Tehran had warned neighbours it would hit American bases if Washington strikes.
With Iran's leadership trying to quell the worst domestic unrest the Islamic Republic has ever faced, Tehran is seeking to deter US President Donald Trump's repeated threats to intervene on behalf of anti-government protesters.
A US official, speaking on condition of anonymity on Wednesday, said the US was pulling some personnel from key bases in the region as a precaution given heightened regional tensions.
"All the signals are that a US attack is imminent, but that is also how this administration behaves to keep everyone on their toes. Unpredictability is part of the strategy," a Western military official told Reuters.
At the White House, however, Trump suggested he was adopting a wait-and-see posture toward the crisis.
Trump told reporters he has been told that killings in the Iranian regime's crackdown on the protests were subsiding and that he believes there is currently no plan for large-scale executions.
Asked who told him that the killings had stopped, Trump described them as "very important sources on the other side".
The president did not rule out potential US military action, saying "we are going to watch what the process is" before noting that his administration had received a "very good statement" from Iran.
Two European officials said US military intervention could come in the next 24 hours. An Israeli official also said it appeared Trump had decided to intervene, though the scope and timing remained unclear.
Britain was also withdrawing some personnel from an air base in Qatar ahead of possible US strikes, The I Paper newspaper reported. The British defence ministry had no immediate comment.
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 unrest 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 Shi'ite clerical rule.
An Iranian official has said more than 2000 people have died. A rights group put the toll at more than 2600.
Iran has "never faced this volume of destruction", Armed Forces Chief of Staff Abdolrahim Mousavi said on Wednesday, blaming foreign enemies.
Iranian authorities have accused the US and Israel of fomenting the unrest, carried out by people it calls armed terrorists.
Trump has openly threatened to intervene in Iran for days, without giving specifics. In an interview with CBS News on Tuesday, he vowed "very strong action" if Iran executes protesters. He also urged Iranians to keep protesting and take over institutions, declaring "help is on the way".
The senior Iranian official, speaking on condition of anonymity, said Tehran had asked US allies in the region to prevent Washington from attacking Iran.
"Tehran has told regional countries, from Saudi Arabia and UAE to Turkey, that US bases in those countries will be attacked" if the US targets Iran, the official said.
Direct contacts between Iranian Foreign Minister Abbas Araqchi and US Special Envoy Steve Witkoff have been suspended, the official said.
The US has forces across the region including the forward headquarters of its Central Command at Al Udeid in Qatar and the headquarters of the US Navy's Fifth Fleet in Bahrain.
The flow of information from inside Iran has been hampered by an internet blackout.
