Britain said Iran seized two oil tankers in the Gulf on Friday and told Tehran to return the vessels or face consequences in the latest confrontation to ratchet up tension along a vital international oil shipping route.

Quoted in a report by Reuters, Iran’s Revolutionary Guards said they had captured the British-flagged Stena Impero, announcing the move two weeks after the British navy seized an Iranian tanker in Gibraltar.

Iran’s semi-official Tasnim news agency said the second vessel, the British-operated Mesdar, had not been seized. It said the ship had been allowed to continue its course after being given a warning over safety and environmental issues.

The Stena Impero and Mesdar changed direction sharply within 40 minutes of each other shortly after entering the Gulf through the Strait of Hormuz, taking up a course toward Iran, Refinitiv tracking data showed.

The data later showed Mesdar changing direction again, heading westward back into the Gulf.

“These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region,” British foreign minister Jeremy Hunt said.

Hunt later said, in comments reported by Sky News, that there would be consequences if Iran did not return control of the ships but said Britain was not considering military options.

US President Donald Trump said he would talk to Britain about the issue, speaking after a war of words earlier on Friday about whether the United States had shot down an Iranian drone in the Strait of Hormuz.

Already strained relations between Iran and the West have become increasingly fraught since the British navy seized Iran’s Grace 1 tanker in Gibraltar on July 4 on suspicion of smuggling oil to Syria in breach of European Union sanctions.

According to Reuters report, Oil prices LCOc1 gained on Friday after the latest spike in tensions along the Strait of Hormuz, through which a fifth of the world’s oil supplies pass.