Trump administration officials acknowledged in closed-door briefings with congressional staff on Sunday that there was no intelligence suggesting Iran planned to attack United States forces first, two people familiar with the matter said.

The US and Israel launched their most ambitious attacks on Iran in decades on Saturday, assassinating Supreme Leader Ayatollah Ali Khamenei, sinking Iranian warships and hitting more than 1,000 targets so far, officials say.

But Sunday’s remarks to Congress appeared to undercut one of the key arguments for the war made by senior administration officials.

They told reporters the day before that US President Donald Trump decided to launch the attacks in part because of indicators that Iranians might strike US forces in the Middle East “perhaps preemptively”.

Trump, one of the officials said, was not going to “sit back and allow American forces in the region to absorb attacks”.

Pentagon briefings lasted more than 90 minutes

Pentagon officials briefed Democratic and Republican staff of several national security committees in both the Senate and the House of Representatives for more than 90 minutes on the unfolding US attack in Iran, White House spokesperson Dylan Johnson said earlier.

In the briefings, administration officials emphasised that Iran’s ballistic missiles and proxy forces in the region posed an imminent threat to US interests, but there was no intelligence about Tehran attacking US forces first, the two sources, speaking on condition of anonymity, told Reuters.

Trump said the attack, which is expected to run for weeks, aimed to ensure Iran could not have a nuclear weapon, contain its missile program and eliminate threats to the United States and its allies.

He has urged Iranians to rise up and topple the government.

Democrats criticise ‘war of choice’

Still, Democrats have accused Trump of waging a war of choice and have taken aim at his arguments for abandoning peace talks that mediator Oman said still held promise.

Trump has argued, without presenting evidence, that Iran was on track to soon secure the ability to strike the US with a ballistic missile.

His missile claim was not backed by US intelligence reports, and appeared to be exaggerated, sources familiar with the reports have told Reuters.

Questions about the justification for the war come as the US military revealed on Sunday the first American casualties of the conflict.

A Reuters/Ipsos poll on Sunday showed 27 per cent of Americans approved of the strikes, while 43pc disapproved and 29pc were not sure.