US President Donald Trump said he is holding off on a planned military strike on Iran at the request of several allies in the Middle East because "serious negotiations" are underway.

The announcement came a day after Trump warned Tehran the "clock is ticking" to strike a deal, his latest threat to end the ceasefire struck in mid-April.

Trump didn't offer details about the planned attack but said in a social media post that he had instructed the US military to be prepared for a possible "full, large-scale assault of Iran."