Iran has shot down a US military spy drone amid as tensions mount in the Gulf of Oman.
Tehran claims its Revolutionary Guard shot down a US drone - identifying it as an RQ-4 Global Hawk used for surveillance - in its airspace.
But the US disputed that account, claiming one of the Navy's MQ-4C Triton high-altitude surveillance drones was taken out by an Iranian surface-to-air missile in international airspace.
A single MQ-4C Triton drone is said to cost $182million (£143million).
Iran said the remote-control aircraft was downed in the southern province of Hormozgan, which is along the Gulf of Oman, while the US insisted that it was shot down over the Strait of Hormuz
The development came after US President Donald Trump's administration accused Tehran of attacking two tankers in the Gulf last week, and the Pentagon accused Iran of trying to shoot down a Reaper drone in the same area.
The Revolutionary Guard claimed: "It was shot down when it entered Iran's airspace near the Kouhmobarak district in the south."
A US military source told Fox News the drone was shot down 17 miles from Iran.
Navy Captain Bill Urban, a spokesman for the US military's Central Command, claimed no US aircraft were flying over Iran on Wednesday.
Last week, the US military claimed Iran tried to use a surface-to-air missile to hit a MQ-9 drone that was carrying out surveillance after the attacks on the Kokuka Courageous and the Front Altair tankers.
Tehran has denied attacking the vessels, and the US has released video allegedly showing the Revolutionary Guard retrieving an unexploded mine from the Kokuka Courageous while it was adrift hours after the attacks.
The outgoing Secretary of Defence Patrick Shanahan announced the deployment of 1,000 more US troops to the region in response to "hostile behaviour".
"I have authorised approximately 1,000 additional troops for defensive purposes to address air, naval, and ground-based threats in the Middle East," Mr Shanahan said in a statement.
"The recent Iranian attacks validate the reliable, credible intelligence we have received on hostile behaviour by Iranian forces and their proxy groups that threaten United States personnel and interests across the region."
The deployment comes after a similar deployment of 1,500 US troops occurred in late May.
A senior Iranian security official said Tehran would "strongly respond" to any violation of its airspace.
The secretary of Iran's Supreme National Security council told the semi-official Tasnim news agency: "Our airspace is our red line and Iran has always responded and will continue to respond strongly to any country that violates our airspace."
Tension between Iran and the US has escalated since last year when Mr Trump withdrew from a 2015 nuclear deal between Tehran and major powers and reimposed sanctions on it.
Iran and its backers in Russia have accused the White House of unnecessarily increasing tensions in the region, starting with Mr Trump's decision.
When he was in London earlier this month, Mr Trump said "there is always a chance" of US military action against Iran and accused Tehran of being "hostile", but he would prefer to come to a peaceful resolution through talks.
Iranian President Hassan Rouhani responded saying he would not "wage war against any nation", but his country is united against the US.
"Iran will not wage war against any nation," he said.
"Despite all of the Americans' efforts in the region and their desire to cut off our ties with all of the world and their desire to keep Iran secluded, they have been unsuccessful.
"The entire Iranian nation is unanimous in confronting the US in the Middle East," he added.
"The end of this battle will see victory of the Iranian nation."
