— Iran threatened Thursday to retaliate against British shipping after the British navy’s seizure in the Mediterranean of a tanker transporting Iranian oil to Syria, drawing Europe deeper into the escalating tensions that risk war between the United States and Iran.

The Iranian Foreign Ministry called the interception of the tanker by Royal Marines near Gibraltar on Thursday an “act of piracy” and said it had summoned the British ambassador to Tehran to complain. Britain says the tanker was detained on the grounds that oil shipments to Syria violate European Union sanctions.

A senior Iranian official posted a tweet saying Iran should retaliate by snatching British ships if Britain refused to release the tanker. “If the U.K. doesn’t return the Iranian tanker, the duty of the responsible authorities is to seize an English tanker,” said Mohsen Rezaee, the secretary of Iran’s Expediency Council and a former commander in the Islamic Revolutionary Guard Corps, Iran’s most powerful military branch.

The seizure marks the first time a European Union country has sought to forcibly apply E.U. sanctions against Syria since they were introduced eight years ago, and it heightened tensions surrounding both the rapidly unraveling Iran nuclear deal and the safety of shipping in the Persian Gulf. 

Though the move was unrelated to the U.S. sanctions targeting Iranian oil imposed by President Trump after he walked away from the 2015 nuclear deal last year, it contributed to the tensions that have spiked in recent weeks following explosions targeting shipping in the Persian Gulf. The United States and Britain have blamed Iran, saying the attacks appear to be an attempt to retaliate for the U.S. sanctions.

European powers have been trying to remain neutral in the hopes of keeping the nuclear deal alive and have promised to find ways to alleviate the severe effects of the U.S. sanctions on Iran’s economy. 

But Iran says they have not done enough, and with a Sunday deadline set by Tehran just two days away — after which it will breach the deal’s limits on its nuclear program — the tanker seizure has only added to Iranian frustrations, said Ali Vaez of the International Crisis Group. 

“At a time when Iran expected Europe to help keep its economy afloat, the timing couldn’t be worse. It will be hard for Iran to resist retaliation,” he said. “We are reaching a stage where the Iranians feel they have to impose a cost on hostile policies toward them, be it by the U.S. through sanctions or by allies who are enforcing the sanctions.”

Iran is unlikely to make a distinction between E.U. sanctions that apply to the supply of Iranian oil to Syria, and U.S. sanctions targeting exports of Iranian oil, Vaez added.

“Iran already believes the Europeans have failed to mitigate the impact of the U.S. sanctions. But it’s one thing not to provide Iran with economic relief. It’s another to help the United States with the sanctions,” he said.

Iranian officials reiterated an accusation by an unnamed Spanish official quoted by the Associated Press that the United States had ordered Britain to detain the ship. The shipment of fuel would violate U.S. sanctions on Syria and those on Iran. British officials deny that any instruction came from the United States.

Iran does not dispute that the oil carried by the vessel was Iranian and was bound for Syria, but officials in Tehran said they questioned the “legality” of the seizure. “Since Iran’s tanker was carrying fuel and oil to Syrian people for solely humanitarian purposes, the measure taken by the British government is not only illegal, but also inhumane,” said Iran’s semiofficial Fars news agency.

The Panamanian-flagged Grace 1 supertanker is anchored off Gibraltar’s coast under the control of British and local authorities pending investigations by local and British authorities, according to the Gibraltar government, which is under British sovereignty. 

Gibraltar’s Supreme Court on Thursday extended the time the tanker may legally be detained from three days to 14, the Gibraltar Chronicle reported. The 28 members of the crew, mostly Indians, Pakistanis and Ukrainians, are being held as witnesses, not criminals, the newspaper said. But if evidence is found on board the ship that its cargo was destined for Syria, arrests may follow, it said.