NEW YORK  -   Donald Trump has said he cancelled an airstrike on Iran with 10 minutes to go because it would not have been proportionate to have killed 150 people in retaliation for the downing of an unmanned US drone.

“We were cocked & loaded to retaliate last night on 3 different sights when I asked, how many will die,” the US president tweeted on Friday. “150 people, sir, was the answer from a General. 10 minutes before the strike I stopped it, not … proportionate to shooting down an unmanned drone.

“I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world. Sanctions are biting & more added last night. Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD!”

It was reported overnight that Trump had given approval for the US military to launch the strikes on Iran as a reprisal for Tehran shooting down the drone, before pulling back at the last minute.

Planes were in the air and ships were in position, but no missiles had been fired when word came to stand down on Thursday night, the New York Times quoted an unnamed official as saying.

US military and diplomatic officials were expecting strikes on a handful of radar and missile sites after the president’s leading national security officials and congressional leaders gathered at the White House, the paper said. The military operation was called off about 7.30pm ET (12.30am BST).

The UK was informed of the US plan for the attack and not told the reprisal raids were off until 10pm (3am BST). On Friday, Reuters reported that Trump had passed a message to Tehran via Oman warning an attack was imminent.

“In his message, Trump said he was against any war with Iran and wanted to talk to Tehran about various issues,” an anonymous Iranian official told the news agency. “He gave a short period of time to get our response, but Iran’s immediate response was that it is up to [the] supreme leader [Ayatollah Ali] Khamenei to decide about this issue.”

A second Iranian official said: “We made it clear that the leader is against any talks, but the message will be conveyed to him to make a decision … However, we told the Omani official that any attack against Iran will have regional and international consequences.”

Oman – along with Japan, Iraq and to a lesser extent Switzerland – has acted as an intermediary for messages between Trump and the Iranian leadership. Khamenei has repeatedly said he will not talk to the US until it lifts economic sanctions, adding that he does not trust Trump’s motives.

The reported contact with Oman suggests the White House might have been involved in brinkmanship with Tehran, but pulled back when Iran did not flinch.

US officials said Trump was known to want talks, but was also a believer in sending mixed messages to keep his adversaries guessing about his next move. Some of his officials, notably the national security adviser, John Bolton, are thought to favour an attack.

One of the targets of the planned strikes was the S-125 Neva/Pechora surface-to-air missile system, Newsweek quoted a Pentagon official as saying. It reported that the US believed the system was behind the drone attack, although Tehran said it had used its 3rd Khordad air defence system, the Iranian equivalent of the Russian Buk system that downed Malaysia Airlines flight MH17 over Ukraine in 2014.

The strikes were seemingly set for early in the day to minimise risk to the Iranian military and civilians.

Trump had earlier appeared keen to calm tensions following the shooting down of the US Global Hawk drone, saying blame might be on a “loose and stupid” Iranian officer acting without authorisation from Tehran.

“We didn’t have a man or woman in the drone. It would have made a big, big difference,” Trump said. Asked how the US would respond, he said: “You’ll find out.”

The downing of the aircraft was the latest in a series of incidents that have raised tensions in the Gulf. Earlier, six oil tankers were damaged in two separate attacks.

The report of the swift reversal came as the US Federal Aviation Administration banned all US airlines and aircraft from flying in Iranian airspace close to where the drone was shot down, due to “heightened military activities” in the region.

On Thursday, Iran’s foreign minister and the US military offered competing graphics showing the drone’s flight path and where it was brought down.

The three European signatories to the Iran nuclear deal – the UK, France and Germany – are expected to shortly issue a demarche to Iran warning they will have to withdraw their commitment to the agreement as soon as the International Atomic Energy Authority (IAEA) declares Iran has breached it.

The demarche is designed as a warning shot to demonstrate that whatever sympathy the EU countries may have with the effect on Iran of US economic sanctions, Tehran cannot expect any leeway to breach the deal.

Iran has said it is incrementally stepping away from the agreement by allowing its enriched uranium stockpile to surpass the 300kg limit from 27 June, but has not said how fast it will then increase the stockpile.

Iran also said that from 7 July, it will increase uranium enrichment to above the 3.75% level set out in the agreement, but again, it has not said how quickly this would later go up.

An EU withdrawal from the deal would likely mean the bloc’s sanctions being reapplied, as well as symbolising the further diplomatic isolation of Iran.

The member states have stuck with the agreement, resisting pressure from Washington to follow the lead of Trump. Prior to the US withdrawal, the EU had been negotiating with Washington to see if the deal could be expanded or include an annexe dealing with Iran’s ballistic missile programme.

The demarche will stress that it will be for the IAEA to determine if its intrusive inspections regime shows there has been a breach.