Iran said on Friday it had "indisputable" evidence a US drone it downed had violated its airspace, as reports emerged in the US that President Donald Trump had approved, then scrapped, retaliatory strikes against Iranian targets because the response was 'not proportionate'.

The downing of the drone — which Washington insists was above international waters but Iran says was within its airspace — has seen tensions between the two countries spike further after a series of attacks on tankers the US has blamed on Tehran.

The head of the Revolutionary Guard’s aerospace division has said Iran had warned the US military surveillance drone several times before launching a missile at it.

Gen. Amir Ali Hajizadeh made the comment in an interview with Iranian state television on Friday. Debris from what Iranian authorities described as pieces of the US Navy RQ-4A Global Hawk lay behind him.

Hajizadeh told state TV: "Unfortunately they did not answer." He added that Iran had collected the debris from its territorial waters.

"US forces in the region were a threat, but they are now an opportunity (for Iran) [...] They do not talk about war with Iran, because they know how susceptible they are."

Reuters cited a Tasnim news agency report, quoting an Iranian Revolutionary Guard commander, who said today that a manned US military P-8 Poseidon reconnaissance jet had accompanied the drone into Iranian airspace but Tehran refrained from shooting it down as well.

Iran said on Friday it had called in the Swiss ambassador, whose country has represented US interests since the severance of diplomatic relations in the aftermath of the Islamic revolution of 1979, to issue a formal protest.

Deputy foreign minister Abbas Araghchi provided the ambassador with "indisputable" evidence the drone had violated Iranian airspace, the foreign ministry said.

"Even some parts of the drone's wreckage have been retrieved from Iran"s territorial waters," Araghchi told the Swiss envoy.

Araghchi "reiterated that Iran does not seek a war and conflict in the Persian Gulf", but warned: "The Islamic Republic of Iran would not hesitate for a moment to decisively defend its territory against any aggression."

The US has asked the United Nations Security Council to meet on Iran behind closed doors on Monday, diplomats said.

"We will brief the council on the latest developments with regard to Iran and present further information from our investigation into the recent tanker incidents," the US mission to UN said in a note to council colleagues, seen by Reuters.

What will be Washington's next move?

Under pressure to respond to the high-stakes incident near the strategic Strait of Hormuz, Trump issued orders for retaliatory strikes on a limited number of Iranian targets, the New York Times reported.

The US was planning to hit "a handful of Iranian targets, like radar and missile batteries" Thursday evening, the newspaper said, citing senior administration officials, but the plan was suddenly aborted in its early stages.

White House and Pentagon officials initially declined to comment, according to the Times. Trump tweeted later on Friday that he had blocked the planned military strike against Tehran because it would not have been a proportionate response to Iran's downing of an unmanned US surveillance drone, adding that more sanctions were imposed late on Thursday.

In a series of early morning tweets, the US president said the military was ready to retaliate last night when he was informed that 150 people would die.

"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," he said, adding: "Sanctions are biting and more added last night."

The US president had appeared to strike a combative tone in his public comments yesterday before rowing back.

"Iran made a very big mistake!" he had tweeted in response to news Iran had shot down the Global Hawk surveillance aircraft.

"This country will not stand for it, that I can tell you," he said later at the White House.

But as the pre-dawn incident whipped up fears of open conflict between the United States and its declared foe Iran — sending crude oil prices soaring — Trump moved swiftly to dial back tensions.

"I find it hard to believe it was intentional, if you want to know the truth," Trump said. "I think that it could have been somebody who was loose and stupid that did it."

The president's mixed message left the world unsure what Washington's next move would be.

Foreign Minister Mohammad Javad Zarif said on Thursday that Iran would go to the United Nations to prove that the drone had entered its airspace before being shot down.

"We don't seek war, but will zealously defend our skies, land and waters," Zarif tweeted.

Iran dies reports of prior warning of retaliation

After unnamed Iranian officials speaking on condition of anonymity told Reuters on Friday that Tehran had received a message from Trump warning of an imminent US retaliatory attack on Iran, Tehran officially denied the reports.

"America did not send any message through Oman for Iran," said Keyvan Khosravi, spokesman for Iran's Supreme National Security Council. "There is no truth in that," he added, quoted by state television.

Khosravi's denial follows a foreign media report which, citing Iranian officials, had said that Oman relayed a message from Trump overnight warning of an imminent US attack unless the Islamic republic agreed to negotiate. News of the message came shortly after the NYT report.

One of the officials who spoke to Reuters said: "In his message, Trump said he was against any war with Iran and wanted to talk to Tehran about various issues. [...] He gave a short period of time to get our response but Iran's immediate response was that it is up to Supreme Leader (Ayatollah Ali) Khamenei to decide about this issue."

A second Iranian official had 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."

US flight restrictions

The US Federal Aviation Administration warned of danger to flights "demonstrated by the Iranian surface-to-air missile shoot-down of a US unmanned aircraft system," and barred American civilian aircraft from the area where it took place "until further notice."

The Pentagon denounced the "unprovoked attack," claiming the navy drone was 34 kilometres (21 miles) from Iran when destroyed by a surface-to-air missile.

It published a map showing the flight path of the drone, which indicated it travelled outside of Iranian waters and included a photograph showing it was at the coordinates (25°57'42"N 56°50'22"E) when it was downed.

Zarif provided different coordinates.

"At 00:14 US drone took off from UAE in stealth mode & violated Iranian airspace," Zarif tweeted. "It was targeted at 04:05 at the coordinates (25°59'43"N 57°02'25"E) near Kouh-e Mobarak.

In a letter to the UN Security Council and UN Secretary General Antonio Guterres, Iran protested against a "dangerous and provocative act by the US military forces against the territorial integrity of the Islamic Republic of Iran."

The drone downing came as Iran was 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 commander of the US Naval Forces Central Command, Sean Kido, said a mine allegedly used in one of the attacks matched Iranian weaponry and that incriminating fingerprints had also been collected.

Options 'running out?'

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

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

A key Republican ally of Trump, Senator Lindsey Graham, said the president's "options are running out".

Asked if he believed the countries were nearing conflict, he replied: "I think anybody would believe that we're one step closer."

One of Trump's biggest opponents, the Democratic speaker of the House of Representatives, Nancy Pelosi, warned that "there's no appetite for wanting to go to war in our country".

Russian President Vladimir Putin, who has close relations with Iran's leadership, said US military retaliation "would be a disaster for the region".