Iranian media announced Friday that Tehran seized Stena Impero, a U.K.-flagged oil tanker, as tensions continue to rise following Britain’s July 4 seizure in Gibraltar of a vessel purportedly carrying Iranian crude to Syria.

Despite comments from unnamed U.S. officials suggesting that Iran had seized a second tanker, Iranian media reported that the Liberian-flagged Mesdar was stopped for safe harbor and environmental questions, and then sent on its way. That vessel is owned by the British Virgin Islands-based New Ocean Shipping Venture Ltd., with an address in Scotland to a company called Norbulk Shipping U.K. Ltd., according to maritime records. Its operator said late Friday that the vessel has resumed its voyage, Reuters reported.

Earlier Friday, Gibraltar authorities extended the detention of Grace 1 for another 30 days. The Grace 1 was full to capacity with oil when it was seized, Gibraltar authorities said at the time. Ayatollah Ali Khameini, Iran’s supreme leader, said this week that Tehran would respond at a time of its choosing to the U.K.’s “piracy” over the Grace 1 seizure.

Grace 1 is owned by Grace Tankers Ltd., a company registered in the Carribean island of Saint Kitts and Nevis. Grace Tankers shares an address with the Singapore-based Iships Management Pte. Ltd, which has served as the vessel’s operator, ship manager and document compliance company since June 2014. Iships Management operates at least four other vessels; all five are owned by companies that share an address with Iships Management. 

The seizure of Stena Impero, by Iran’s Islamic Revolutionary Guard Corps-Navy Force (IRGC-NF), was aired on state TV and a photo of the vessel was published by the government’s official newspaper. Stena Impero was headed to Saudi Arabia before abruptly changing course in the Strait of Hormuz, according to media reports.

The Stena Impero was violating innocent passage and undermining the safety of shipping in the area, Iranian media reported, citing an official from the Iranian Ports and Maritime Organization. The vessel switched off its location transponder and was causing severe pollution and damage to the Persian Gulf, the report said.

President Donald Trump said he would talk to the U.K. after the announcement of the seizure. Oil prices jumped late Friday on the news of the seizure.

Stena Bulk and Northern Marine Management, the owner and operator of Stena Impero, said in a joint statement confirming that the vessel was approached by unidentified small crafts and a helicopter during transit through the strait while it was in international waters. “We are presently unable to contact the vessel, which is now heading north toward Iran,” the statement said.

The U.K. had offered to return Grace 1 earlier this week if Iran guaranteed the oil aboard the vessel would not be delivered to Syria in violation of sanctions. U.K. Foreign Secretary Jeremy Hunt, following reports that the Mesdar was also seized, said he was “extremely concerned” and told reporters he was convening a national security meeting to discuss the issue.

“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,” Hunt said, according to a Reuters report. 

Analysts from the Iran team contributed to this report.