(Adds U.S. maritime security initiative)
* Iran's Guards say they capture Stena Impero tanker
* Britain's Hunt says extremely concerned
* Oil prices gain on Gulf tension
* Trump says no doubt that U.S. downed Iranian drone
* GRAPHIC-Map of tanker's route: https://tmsnrt.rs/32EB5Yv
By Parisa Hafezi and Steve Holland
DUBAI/WASHINGTON, July 19 (Reuters) - Iran said it had seized a British oil tanker in the Strait of Hormuz on Friday but denied Washington's assertion that the U.S. Navy had downed an Iranian drone nearby this week, as tensions in the Gulf region rose again.
Britain said it was urgently seeking information about the Stena Impero tanker, which had been heading to a port in Saudi Arabia and suddenly changed course after passing through the strait at the mouth of the Gulf.
The tanker's operator, Stena Bulk, said in a statement the ship was no longer under the crew's control and could not be contacted.
Iran's state news agency IRNA quoted a military source as saying the vessel had turned off its tracker, ignored warnings from the Revolutionary Guards and was sailing in the wrong direction in a shipping lane.
"We will respond in a way that is considered but robust and we are absolutely clear that if this situation is not resolved quickly there will be serious consequences," British Foreign Secretary Jeremy Hunt told reporters.
Hunt said Britain was "not looking at military options, we are looking at a diplomatic way to resolve the situation."
Stena Bulk said the ship was "in full compliance with all navigation and international regulations."
"There are 23 seafarers onboard of Indian, Russian, Latvian and Filipino nationality," said Erik Hanell, president and chief executive of Stena Bulk. He said there had been no reported injuries.
The Revolutionary Guards did not capture a second tanker - the British-operated, Liberian-flagged ship Mesdar - in the Gulf, Iran's semi-official Tasnim news agency reported, quoting regional military sources.
The Mesdar had turned sharply north toward Iran's coast on Friday afternoon but then changed course again and headed westward away from Iran, according to Refinitiv tracking data.
Relations between Washington and Tehran worsened last year when U.S. President Donald Trump abandoned a 2015 nuclear deal between world powers and Iran. Under the pact, Iran agreed to restrict nuclear work, long seen by the West as a cover for developing atomic bombs, in return for lifting sanctions. But sanctions have been imposed again, badly hurting Iran's economy.
Trump said he would talk to Britain about Friday's seizure, which drove oil prices up above $62 a barrel.
The United States has blamed Iran for a series of attacks since mid-May on shipping around the Strait of Hormuz. Tehran rejects the allegations.
The incidents have increased international concern that both sides could blunder into a war in the strategic waterway, which is vital to world oil supplies.
Iran shot down a U.S. drone in the Gulf in June and British naval forces seized an Iranian tanker in Gibraltar on July 4 on suspicion of smuggling oil to Syria.
The U.S. military said on Friday that unarmed surveillance aircraft were in international airspace, monitoring the Strait of Hormuz and had been in contact with U.S. ships in the area.
"We have patrol aircraft operating in international airspace monitoring the situation within the Strait of Hormuz," said Lieutenant Colonel Earl Brown, a U.S. Central Command spokesman.
A U.S. official, speaking on condition of anonymity, said about 500 U.S. military personnel would be deployed to Saudi Arabia, part of an increase in troops to the Middle East announced by the Pentagon last month.
Iran refuted a U.S. assertion that it shot down an Iranian drone on Thursday in the Strait of Hormuz because it had come too close to a U.S. warship.
Trump said on Thursday that the U.S. Navy warship Boxer, an amphibious assault ship, had destroyed an Iranian drone because it had flown to within 1,000 yards (914 meters) of the ship in a "provocative and hostile action." A U.S. official said the drone was brought down by electronic jamming.
"No doubt about it, no. We shot it down," Trump said.
He said the United States hopes "for their sake they don't do anything foolish. If they do they will pay a price like nobody has ever paid a price."
But Iran said all of its drones were accounted for.
"All drones belonging to Iran in the Persian Gulf and the Strait of Hormuz ... returned safely to their bases after their mission of identification and control," Abolfazl Shekarchi, a senior Iranian armed forces spokesman, was quoted by Tasnim as saying.
He said there was no report of any "operational response" by the Boxer and Iran's state television broadcast a video showing aerial views of ships that it said disproved the U.S. claim.
The television station said the footage, which came from the Revolutionary Guards, and the timing notations indicated the pilotless aircraft was still filming after Washington said it had been downed.
U.S. officials briefed members of the Washington diplomatic corps on Friday about a multinational maritime security initiative being developed to de-escalate tensions in the Gulf.
A senior Pentagon told Reuters earlier this week the United States was not aiming to set up a military coalition against Iran but simply "shining a flashlight" in the region to deter attacks on commercial ships.
(Reporting by Steve Holland in Washington and Parisa Hafezi in Dubai; additional reporting by Idrees Ali in Washington, Reuters staff in Dubai and Kate Holton and Jonathan Saul in London; writing by Alistair Bell; editing by Grant McCool, Cynthia Osterman and Sonya Hepinstall)
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