Washington (AFP) - President Donald Trump said Friday the United States was "cocked & loaded" to strike Iran but pulled back at the last minute because it would not have been a "proportionate" response to Tehran shooting down an American drone.

The downing of the drone -- which Iran insists violated its airspace, a claim Washington denies -- has seen tensions between the countries spike after a series of attacks on tankers the US has blamed on Tehran.

Under pressure to respond to the high-stakes incident near the strategic Strait of Hormuz, Trump said the US was prepared to hit "3 different sites" Thursday night but that he scrapped 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," the president tweeted, saying he concluded 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 strikes against Iran, and that no planes were in the air.

"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, adding that he did not want war with Iran, but if it came to pass, there would be "obliteration like you've never seen before."

The US president had struck a combative tone in initial comments Thursday about Iran shooting down the Global Hawk surveillance aircraft, but as the pre-dawn incident whipped up fears of open conflict, Trump moved to dial back tensions.

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

And it denied a report that Trump had warned it via Oman of an impending attack unless it was willing to negotiate.

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The US special representative on Iran, Brian Hook, accused Tehran of rejecting diplomatic overtures to deescalate regional tensions.

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

Amid the tensions, Trump late Friday nominated Mark Esper to be Secretary of Defense, one of the most powerful posts in the US government and a key advisor to the president as Washington navigates the dispute with Iran.

The US hasn't had a full defense secretary since the resignation of James Mattis in December last year. Esper, who this week was elevated to acting Pentagon chief, still needs to be confirmed by the Senate.

On the streets of Tehran, anxiety over a potential war was added to residents' concerns over crippling US sanctions.

"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 withheld his last name.

- 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.

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 sensitive Strait of Hormuz.

The Pentagon says the Global Hawk drone -- one of the most expensive pieces of equipment in the US arsenal, costing over $120 million apiece -- was 34 kilometers (21 miles) from Iran when destroyed by a surface-to-air missile in an "unprovoked attack."

It published a map of the drone's flight path indicating it avoided Iranian waters, but Tehran provided its own map showing the aircraft inside its territory when it was downed 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.

Trump has repeatedly said he does not favor war with Iran unless it is to stop the country getting a nuclear weapon -- something Iranian leaders insist they are not pursuing.

But critics say his policy of "maximum pressure" -- including abandonment of an international deal to regulate Iran's nuclear activities, economic sanctions and deployment of extra troops to the region -- make war ever more likely.

US House Speaker Nancy Pelosi warned Friday of "an extremely dangerous and sensitive situation," but said she was "absolutely" pleased with Trump's decision not to carry out the strike.

Candidates for the Democratic presidential nomination also expressed concerns, with Senator Bernie Sanders warning war with Iran would "lead to endless conflict in the region," and Senator Elizabeth Warren urging Washington "to step back from the brink of war."