US President Donald Trump says he is not looking for war with Iran but has warned that if a conflict did occur, it would result in "obliteration".

Key points: US President Donald Trump said he never gave the final approval for retaliatory strikes

US President Donald Trump said he never gave the final approval for retaliatory strikes Iran says it warned the unmanned drone several times before downing it with a missile

Iran says it warned the unmanned drone several times before downing it with a missile It also says its military avoided shooting down a nearby manned US aircraft

"I'm not looking for war, and if there is it'll be obliteration like you've never seen before. But I'm not looking to do that," Mr Trump told NBC News in an interview a day after he aborted a planned air strike against Iranian targets in retaliation for Tehran shooting down a US drone.

Mr Trump said the US was within 10 minutes of conducting strikes against Iran on Thursday (local time) when he cancelled the operation.

He told NBC News that he never gave a final order — planes were not yet in the air but would have been "pretty soon".

He said military officials came to him about 30 minutes before the strikes were to be launched and asked him for his final approval.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 42 seconds 42 s Wreckage that Iran says was retrieved after it shot down a US drone

Before signing off, he said he asked how many Iranians would be killed and was told approximately 150.

"I thought about it for a second and I said, 'You know what? They shot down an unmanned drone, plane — whatever you want to call it — and here we are sitting with 150 dead people. That would have taken place probably within half an hour after I said 'go ahead'. And I didn't like it. I didn't think it was proportionate," he said.

Mr Trump's assertion that he learned only at the last minute of his military advisers' casualty estimate does not align with the usual way a president is briefed on military attack options.

An assessment of the likelihood of casualties, whether civilian or military, and a broad estimate of the number, normally are a major element of each option provided to the commander in chief.

General Amir Ali Hajizadeh said the US drone was warned 10 minutes before the attack. ( AP: Meghdad Madadi/Tasnim News Agency )

Iran addressed the subject of casualties, too. General Amir Ali Hajizadeh, the head of the Revolutionary Guard's aerospace division, said that a US spy plane with around 35 crew members was flying close to the unmanned US Navy RQ-4A Global Hawk that was shot down, but that Iran chose not to target the manned aircraft.

He said Iran warned the drone several times before downing it with a missile.

The President's decision to call off the attack is a reminder that despite the escalation in tensions between Washington and Tehran in recent weeks, there is a realisation that military action, once underway, can quickly lead to unintended consequences, including large-scale war.

Mr Trump has said repeatedly he does not want war with Iran, but he has offered little insight into his strategy, beyond inviting Iran's leaders to call him to reopen nuclear negotiations.

His administration last year pulled out of the 2015 international agreement intended to curb the Iranian nuclear program, an agreement he strongly criticised as ineffective during his presidential campaign. He demanded negotiations for a new agreement, but there have been none.

Pressuring Iran, he launched a campaign of increasing economic pressure against the Islamic Republic, including cutting off its oil export revenues.

AP