Trump repeatedly said he doesn't favor war with Iran unless it is to stop it getting a nuclear weapon

U.S. President Donald Trump played down Iran's downing of a U.S. military drone on Thursday, saying he suspected it was shot by mistake and that "it would have made a big difference" to him if the remotely-controlled aircraft had been piloted.

While the comments appeared to suggest Trump was not eager to escalate the latest in a series of incidents with Iran, he also warned that: "This country will not stand for it."

Tehran said the unarmed Global Hawk surveillance drone was on a spy mission over its territory but Washington said it was shot down over international airspace.

"I think probably Iran made a mistake - I would imagine it was a general or somebody that made a mistake in shooting that drone down," Trump told reporters at the White House.

"We had nobody in the drone. It would have made a big difference, let me tell you, it would have made a big, big difference" if the aircraft had been piloted, Trump said as he met Canadian Prime Minister Justin Trudeau in the Oval Office.

The United States, which called the event an "unprovoked attack" in international air space, is pursuing a campaign to isolate Iran to contain its nuclear and ballistic missile programmes and limit its role in regional wars.

The incident was the latest in an escalating series of incidents in the Gulf region, a critical artery for global oil supplies, since mid-May including explosive strikes on six oil tankers as Tehran and Washington have edged towards confrontation.

It was unclear how the United States might respond and U.S. House of Representatives Speaker Nancy Pelosi, the top Democrat in Congress, said Washington had no appetite for war with Iran.

"It's hard to believe it was intentional, if you want to know the truth. I think that it could have been somebody who was loose and stupid that day," Trump said, referring to the drone shooting.

The Trump administration called top congressional leaders to the White House for a briefing later on Thursday on Iran, a source with knowledge of the meeting said.

Iran has denied involvement in the tanker attacks, but global jitters about a new Middle East conflagration disrupting oil exports have triggered a jump in crude prices.

Saudi Arabia, Washington's main gulf ally, said Iran had created a grave situation with its "aggressive behaviour" and the kingdom was consulting other Gulf Arab states on next steps.

Tensions with Iran flared with Trump's withdrawal last year from a 2015 nuclear accord with Iran and have worsened as Washington imposed fresh sanctions to throttle Tehran's vital oil trade. Iran retaliated earlier this week with a threat to breach limits on its nuclear activities imposed by the deal.

'SPY' DRONE

Iranian state media said the "spy" drone was brought down over the southern Iranian province of Hormozgan, which is on the Gulf, with a locally made "3 Khordad" missile.

A U.S. official said the drone had been downed in international air space over the Strait of Hormuz, through which about a third of the world's seaborne oil exits the Gulf.. Earlier, a U.S. official had described the drone as an MQ-4C Triton, a similar aircraft.

A U.S. official said the drone, an RQ-4A Global Hawk, had been downed in international air space over the Strait of Hormuz, through which about a third of the world's seaborne oil exits the Gulf.. Earlier, a U.S. official had described the drone as a Triton, a similar aircraft.

Navy Captain Bill Urban, a spokesman for the U.S. military's Central Command, said Iran's account that the drone had been flying over Iranian territory was false.

"This was an unprovoked attack on a U.S. surveillance asset in international air space," Urban said. The drone, he added, was downed over the Strait of Hormuz at approximately 2335 GMT - in the early morning hours of local time in the Gulf.

Separately, a U.S. official told Reuters on condition of anonymity the debris field from the drone was in international waters in the Strait and U.S. naval assets have been dispatched to the area.

Iran's foreign ministry said the drone had violated Iranian air space and warned of the consequences of such "illegal and provocative" measures.

Independent confirmation of the drone's location when it was brought down was not immediately available.

A Iranian Revolutionary Guards statement said the drone's identification transponder had been switched off "in violation of aviation rules and was moving in full secrecy" when it was downed, Iranian state broadcaster IRIB reported.

IRANIAN "RED LINE"

"Our air space is our red line and Iran has always responded and will continue to respond strongly to any country that violates our air space," Ali Shamkhani, secretary of the Supreme National Security Council, told Iran's Tasnim news agency.

The Global Hawk's manufacturer, Northrop Grumman, says on its website that it can fly for over 24 hours at a time at altitudes higher than 10 miles (16 km).

Upping the ante in its dealings with Iran, Washington said on Monday it would deploy about 1,000 more troops, along with Patriot missiles and manned and unmanned surveillance aircraft, to the Middle East on top of a 1,500-troop increase announced after the May tanker attacks.

The Trump administration sought on Wednesday to rally global support for its pressure on Iran by displaying limpet mine fragments it said came from an oil tanker damaged in the June 13 attacks, saying the ordnance closely resembled mines publicly displayed in Iranian military parades.

European diplomats have said more evidence is needed to pinpoint responsibility for the tanker strikes.

U.S. sanctions have hammered Iran's economy, scuttling its oil exports and barring it from the dollar-dominated global finance system. That undoes the promise of trade rewards from the 2015 deal to curb its nuclear ambitions.

Trump has sent forces including aircraft carriers, B-52 bombers and troops to the Middle East over the past few weeks. Iran said last week it was responsible for the security of the Strait of Hormuz, calling on American forces to leave the Gulf.

Tehran has also said it will shortly suspend compliance with the nuclear deal's curbs on its uranium enrichment, meant to block any pathway to nuclear weapons capability, and threatened to disrupt oil shipments through the Strait of Hormuz.