Iran’s Revolutionary Guard Corps seized a British oil tanker in the Strait of Hormuz amid soaring tensions in one of the world’s critical energy chokepoints. Brent climbed more than 1.3 per cent on the news.

U.S. President Donald Trump told reporters at the White House that he will be “working with the U.K.” on the incident. “This only goes to show what I’m saying about Iran: trouble, nothing but trouble.”

A spokesperson for the British government said it was urgently seeking further information and assessing the situation.

A second oil tanker in the area, the Liberian-flagged Mesdar, operated by a U.K. company, appeared to turn toward the Iranian coast, according to ship-tracking data. It had been heading toward Saudi Arabia. Trump told reporters on Friday afternoon the administration had been hearing conflicting reports about whether a second tanker had been taken.

Allahmorad Afifipour, the head of the Ports and Maritime Organization in Iran’s Hormozgan province, said the group had received reports the first tanker “had an incident with another vessel,” but offered no specifics. He said the ship was empty and “destined for one of the Persian Gulf littoral states.”

Friday’s incident was at least the second Iranian move against a U.K. ship in just over a week. On July 11, the British navy intervened to stop Iran from blocking a commercial oil tanker leaving the Persian Gulf. That followed an incident in early July when U.K. forces seized a tanker off Gibraltar that was suspected of carrying Iranian oil to Syria. Iran denied the vessel was heading to Syria and has since vowed to retaliate.

The tit-for-tat actions signal greater instability and potential conflict in a region where one-third of the world’s seaborne crude and fuels pass through. The U.S. and Iran have been locked in a standoff over American sanctions on Iranian oil following President Donald Trump’s withdrawal from the 2015 nuclear accord last year.

“These incidents in isolation are not especially alarming,” former U.S. National Security Advisor Susan Rice said at a conference in Aspen, Colorado. But, she added, “In the aggregate they are, given that we’re dealing with players that have little interest in deescalating.”

Recent weeks have seen a spate of attacks on oil tankers and other flare ups in the Gulf region. On Thursday, the U.S. said it downed an Iranian drone that was endangering the naval ship USS Boxer, a claim Iran has rejected. In June, Trump said he called off a retaliatory strike on Iran following Tehran’s move to shoot down an American drone.

On Friday, the Swedish owner of the Stena Impero said it “was approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters” at about 4pm London time. “We are presently unable to contact the vessel which is now heading north towards Iran.”

The 49,683 deadweight ton tanker, which has 23 workers aboard and typically carries refined products, was last heading to Jubail, Saudi Arabia, according to ship-tracking data.

Following the Gibralatar seizure earlier this month, British forces had been escorting ships that were flying the nation’s flag, and it appeared oil tankers registered in the Isle of Man, Gibraltar and the U.K. had largely vacated the gulf.