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 advisor 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."