US President Donald Trump said on Monday he had called off a planned US military strike on Iran scheduled for Tuesday, citing a "very good chance" of reaching a deal to end the war.

In a post on Truth Social, Trump said Gulf allies, including Qatar, Saudi Arabia and the United Arab Emirates, had urged Washington to delay the attack because of progress in negotiations with Iran.

He added that the US military has been instructed to remain prepared for a "full, large scale assault" if talks fail.

"There seems to be a very good chance that they can work something out," Trump told reporters at the White House. "If we can do that without bombing the hell out of them, I'd be very happy."

Earlier, the US president had declared the fragile six-week ceasefire as being "on life support."

Tehran's nuclear program in focus

Iran confirmed that it had conveyed a new proposal to Washington through Pakistan.

Details of the proposal were not disclosed, though a senior Iranian source told Reuters news agency it largely resembled an earlier offer that Trump dismissed last week as "garbage."

One of the major sticking points is Washington's demand that Tehran halt its nuclear program and lift its de facto blockade of the Strait of Hormuz, a key waterway through which one-fifth of the world's oil and liquefied natural gas flows.

For its part, Iran is seeking compensation for war damage as well as an end to all hostilities, including Israeli strikes in Lebanon.

Iran remains defiant

Meanwhile, Iranian officials remained defiant, warning against further US or Israeli attacks and saying Tehran was prepared to respond forcefully to any renewed aggression.

Iran's top joint military command, Khatam al-Anbiya, said the country's armed forces are "ready to pull the trigger" if the US begins new strikes, according to Iran's Tasnim news agency.

The episode is the latest sign that the ceasefire between the US and Iran (reached early April) is under strain as both sides struggle to agree to terms to end the roughly 11-week conflict.