US President Donald Trump said Friday that he was “cocked & loaded” to launch strikes against Iran but had a change of heart at the last minute, concluding that it would not have been a “proportionate” response to Tehran downing an American drone.

Tensions between the two countries have risen dramatically following a series of attacks recently on oil tankers that Washington has blamed on Tehran, and the downing on Thursday of an American drone that Iran insists violated its airspace, a claim the Americans deny.

Read: Iran shoots down US ‘spy drone’ in Persian Gulf

Under increasing pressure to respond to the drone incident near the strategically vital Strait of Hormuz, Trump said the US was prepared to hit “3 different sites” Thursday night but that he called off the strikes “10 minutes” before they were to have been launched.

“I asked, how many will die. 150 people, sir, was the answer from a General,” Trump tweeted, adding that he did not think it would not have been “proportionate to shooting down an unmanned drone.”

According to excerpts of an interview with NBC’s Meet the Press conducted Friday morning at the White House, Trump said he had not given final approval to launch strikes against Iran, and that no warplanes were airborne.

“But they would have been pretty soon. And things would have happened to a point where you wouldn’t turn back or couldn’t turn back,” he said.

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 American officials believed the system was responsible for the drone attack.

The British military was informed of the planned military strike and was told in the early hours that the operation was being aborted. The attack could have threatened British maritime assets in the region.

On Thursday, Trump’s response to Iran shooting down the Global Hawk surveillance aircraft was initially combative, but as the pre-dawn international incident began to raise serious fears of open conflict, he moved to alleviate tensions.

Iran vowed Friday to defend its borders after downing the drone, with the commander of the aerospace arm of its elite Islamic Revolutionary Guard Corps saying the aircraft was warned twice before it was engaged as it flew over the Gulf of Oman.

Tehran denied a report that Trump contacted Iran via a mediator – Oman – demanding that it agree to talks, suggesting Washington may have been using the threat of a strike as a means of brinkmanship, The Guardian reported.

Washington’s special representative on Iran, Brian Hook, accused Tehran of dismissing diplomatic overtures to defuse the situation.

“Iran needs to meet diplomacy with diplomacy, not military force,” Hook told reporters in Saudi Arabia.

Trump and Saudi Crown Prince Mohammed bin Salman on Friday discussed the “threat” posed by Tehran, and the US requested that a UN Security Council meeting on Iran be convened on Monday.

The United Nations called for calm. According to a UN spokeswoman in Geneva, Secretary-General António Guterres said, “I have only one strong recommendation: nerves of steel.”

Many American security experts, including former presidential envoy Brett McGurk, called on the US president to take advantage of his rethink to formulate a consistent strategy on the Islamic republic, including clarifying whether he is seeking regime change or instead a reopening of the 2015 Iran nuclear agreement from which Washington pulled out. McGurk said Trump’s sudden pull-back was the first time the White House had taken its Iran strategy off auto-pilot, The Guardian reported.

Mood in Tehran

On the streets of Tehran, residents, who already feeling the pain of crippling US sanctions, were visibly anxious, AFP reported.

“For me, the situation is already worrying because the economic state of the country is bad, and the possibility of war frightens me,” said Amir, a shopkeeper who did not divulge his surname.

Flights rerouted

Iran said Friday it had presented the Swiss ambassador, whose country represents US interests in Iran, with “indisputable” evidence the drone violated Iranian airspace.

Iranian Deputy Foreign Minister Abbas Araghchi told the envoy “Iran does not seek a war and conflict in the Persian Gulf” while warning it would “decisively defend its territory against any aggression.”

The US Federal Aviation Administration has barred American civilian aircraft from the area “until further notice,” and major non-US airlines including British Airways, KLM, Lufthansa, Qantas, Emirates and Etihad said they too were altering flight paths to avoid the volatile Strait of Hormuz.

Global Hawk

The Pentagon says the Global Hawk drone – one of the most expensive pieces of hardware in its vast arsenal, costing over $120 million apiece – was 34 kilometers from Iran when it was taken down by a surface-to-air missile in what it claims was an “unprovoked attack.”

It published a map of its flight path indicating it avoided Iranian waters, and a photograph showing its coordinates when it was hit.

Iranian Foreign Minister Mohammad Javad Zarif provided different coordinates for the downing of the drone by a domestically-manufactured Khordad 3 air defense battery.

The shootdown came with Iran already accused by Washington of carrying out attacks on tankers in the congested shipping lanes heading out of the Gulf through the Strait of Hormuz.

Tehran denies any involvement.

The US president has repeatedly said he does not favor war with Iran unless it is to stop the Islamic republic from obtaining a nuclear weapon – something Tehran insists it is not pursuing.

However, critics say his policy of “maximum pressure” – including withdrawing from an international agreement to regulate Iran’s nuclear activities, crippling economic sanctions and deployment of extra troops to the region – make the outbreak war increasingly likely.

US House Speaker Nancy Pelosi on Friday expressed alarm Friday over “an extremely dangerous and sensitive situation.”

Senator Elizabeth Warren of Massachusetts, who is running for the Democratic presidential nomination, blasted the US president.

“Donald Trump promised to bring our troops home,” Warren wrote late Thursday night on Twitter. “Instead he has pulled out of a deal that was working and instigated another unnecessary conflict. There is no justification for further escalating this crisis – we need to step back from the brink of war.”

Due to the sometimes capricious US president’s contradictory stance on US involvement in conflicts, it is difficult for analysts to predict how he will proceed in the Strait of Hormuz.

“He has two instincts. One is caution, believing that endless, repeated wars have been too costly for the United States,” said Rob Malley, a former adviser to the Obama administration who heads the International Crisis Group. “The other instinct is to look like someone strong, who can’t be pushed around.”

David Rothkopf, the author of two histories of the National Security Council, believes the US president probably called off the strikes simply because he was spooked. He said on Twitter that the fact that Trump “blinked” in the face of Iran’s aggression was “not a sign of restraint so much as evidence of indecision and bumbling – the situation remains very dangerous and prone to accidental escalation and/or spinning out of control.”

– with additional reporting by AFP, The Guardian, Newsweek, The New York Times and Reuters