Iran has shot down a US military drone it said was on a spy mission over its territory but Washington said the aircraft was targeted in international air space in "an unprovoked attack".
Thursday's incident fanned fears of wider military conflict in the Middle East as US President Donald Trump pursues a campaign to isolate Iran over its nuclear and ballistic missile programmes and role in regional wars.
It 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 slid towards confrontation.
Iran has denied involvement in any of the attacks, but global jitters about a new Middle East conflagration disrupting oil exports have triggered a jump in crude prices.
Tensions flared with the US pullout last year from world powers' 2015 nuclear accord with Iran, and have worsened as Washington imposed fresh sanctions on Tehran's vital oil trade and Iran retaliated with a threat to breach limits on its nuclear activities imposed by the deal.
Upping the ante, Washington said on Monday it would deploy about 1000 more troops, along with Patriot missiles and manned and unmanned surveillance aircraft, to the Middle East on top of a 1500-troop increase announced after the May tanker attacks.
Sepah News, the website of Iran's elite Revolutionary Guards Corps (IRGC), said the "spy" drone was brought down over the southern Iranian province of Hormozgan, which is on the Gulf.
A US official said the drone was a US Navy MQ-4C Triton and that it 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..
Navy Captain Bill Urban, a spokesman for the US 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 US 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.
Independent confirmation of the drone's location when it was brought down was not immediately available.
A 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.
The Iranian Foreign Ministry condemned what it called a violation of Iranian air space by the drone and warned of the consequences of such "illegal and provocative" measures.
Brigadier General Hossein Salami, commander in chief of the Revolutionary Guards, warned against any aggression and said the drone's downing carried a "clear message" to Iran's arch-enemy.
"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 MQ-4C Triton's manufacturer, Northrop Grumman, says on its website that the Triton can fly for over 24 hours at a time at altitudes higher than 10 miles (16 km), with an operational range of 8,200 nautical miles.
It was not immediately known whether the Triton was armed - like drones that have been used by the US military to eliminate al Qaeda militants in Yemen's conflict.
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 looked Iranian in origin.
European diplomats have said more evidence is needed to pinpoint responsibility for the strikes on the tankers.
Australian Associated Press