A former top US military adviser warned Sunday that tensions with Iran “could spin out of control” after President Donald Trump’s last-minute cancelation of air strikes on the Islamic republic.

Washington and Tehran have traded accusations since Iran shot down a US spy drone last week, prompting a plan for retaliatory strikes that was shelved when Trump decided the resulting mass casualties would not be “proportionate.”

“My biggest concern is the president is running out of room, running out of options and while rhetoric goes back and forth on how close we came to hitting Iran just the other day, that this thing could spin out of control,” former chairman of the Joint Chiefs Mike Mullen told ABC’s This Week.

“The last thing in the world we need right now is a war with Iran.”

Trump has labeled Iran a danger and in May last year pulled the US out of an international accord on rewarding the country for allowing verification of its nuclear industry.

Trump has repeatedly sought to downplay moments of tension, repeating his reluctance to see the dispute escalate to military conflict.

He has announced new sanctions beginning Monday and US cyber forces reportedly struck Iranian military computer systems.

But some of his closest aides, such as national security adviser John Bolton and Secretary of State Mike Pompeo, are said to favor a far more muscular US strategy.

Mullen, who served under George W Bush and then Barack Obama from 2007 to 2011, said politicians need to use diplomacy to prevent Iran’s nuclear ambitions.

“That’s our system here and I think the politicians need to figure out a way to achieve the objective, which is Iran without a nuclear weapon – and, from my perspective, without regime change, without going to war,” he said.

Trump tweeted on Friday that US forces were “cocked and loaded” to retaliate after the downing of the drone but that he called them back in order to avoid mass casualties.

Republican House Armed Services ranking member Mac Thornberry, was among a delegation of congressional leaders being briefed at the White House as events unfolded.

“If they go back to mining tankers, shooting at American aircraft, the sort of pattern of activity we’ve seen since April, then obviously the president has a whole range of additional responses that he could employ,” he told This Week co-anchor Martha Raddatz.

“But he’s given himself a lot of headroom, if you will.”

But Senator Cory Booker, a Democratic presidential candidate and member of the Foreign Relations Committee, told the show the president was “taking a belligerent course of escalation and provocation with Iran.”

Potentially exacerbating the situation, Trump said Saturday that the United States would impose “major” new sanctions on Iran in two days.

“We are putting major additional Sanctions on Iran on Monday,” Trump tweeted.

“I look forward to the day that Sanctions come off Iran, and they become a productive and prosperous nation again – The sooner the better!”

Earlier, before heading to Camp David for meetings with his advisers on the situation, Trump said he would be Iran’s “best friend” and that the Islamic republic could be a “wealthy” country if it renounced nuclear weapons.

“We’re not going to have Iran have a nuclear weapon,” Trump told reporters outside the White House.

“When they agree to that, they’re going to have a wealthy country. They’re going to be so happy, and I’m going to be their best friend. I hope that happens.”

“Let’s make Iran great again,” he added, tweaking for the occasion his main domestic political mantra.

Meanwhile, US Secretary of State Mike Pompeo said Sunday he will visit Saudi Arabia and the United Arab Emirates for talks on Iran’s downing of the US drone.

“We’ll be talking with them about how to make sure that we are all strategically aligned and how we can build out a global coalition” on Iran, he said.

Pompeo said he would stop in the two countries on his way to India, where he begins a visit on Tuesday.

Speaking to reporters as he left Washington, Pompeo called Saudi Arabia and United Arab Emirates “great allies in the challenge that Iran presents.”

Pompeo denounced a map that was released by Iranian Foreign Minister Mohammad Javad Zarif that purported to show a spy drone encroaching its airspace in late May and urged media not to find it credible.

“You’ve seen that child-like map that Foreign Minister Zarif put out that contrasts with the excellence and professionalism of America’s military and intelligence services,” Pompeo said.

Pompeo said that Iran’s map “should leave no doubt in anyone’s mind about where that unarmed vehicle was – it was flying in international airspace.”

“We shouldn’t let the Iranians have one moment where any reporter would write that there is even a credible response to the data set that the Americans have put forward,” he said, accusing Iran of “sowing disinformation in lots of lots of places.”

But Pompeo reiterated Trump’s offer of dialogue to improve relations with Iran, which the US administration has sought to isolate through harsh sanctions.

“We’re prepared to negotiate with no preconditions. They know precisely how to find us,” Pompeo said.

“And I am confident that at the very moment they’re ready to truly engage with us, we will able to begin this conversation,” he said.

“I’m looking forward to the day,” he added.