On Thursday, a deployment of British Marines stormed an Iranian oil tanker accused of carrying crude oil to Syria in breach of sanctions against the Assad regime. Now, a senior Iranian military commander has threatened to seize a British ship in retaliation.

British Marines helped Gibraltarian police seize the 330-meter Grace-1 early Thursday after it was suspected of carrying oil from Iran to Syria, in breach of European Union sanctions against the Assad regime. The seizure set off a furious response from Tehran, which called it “piracy” and accused the British government of doing the bidding of the U.S.

On Friday, Maj. Gen. Mohsen Rezaei, a commander in Iran’s Islamic Revolutionary Guard Corps, tweeted that if the Grace isn’t released, Tehran should respond with its own seizure.

“If Britain does not release the Iranian oil tanker, it is the authorities duty to seize a British oil tanker,” he wrote. “Islamic Iran in its 40-year history has never initiated hostilities in any battles, but has also never hesitated in responding to bullies.”

Iranian Foreign Ministry spokesman Abbas Mousavi said that Britain’s ambassador to Tehran, Rob Macaire, had been formally summoned for a complaint about the seizure of the tanker, which he said could further inflame tensions in the Persian Gulf.

The standoff comes at a particularly volatile moment in the relationship between Iran and the West, after Tehran announced it had deliberately exceeded uranium enrichment limits set down in the ailing 2015 nuclear accord, which the U.S. abandoned last year. The U.S., which has since pursued a policy of “maximum pressure” via sanctions on Tehran, accused Iran of carrying out explosive attacks on two oil tankers in the Strait of Hormuz last month. Tehran denied the allegations, accusing the U.S. of “warmongering.”

Fabian Picardo, the chief minister of Gibraltar, a British overseas territory on Spain's southern tip, said his officials stopped the tanker because it was believed to be destined for Syria’s Banyas refinery, which is owned by an entity that’s subject to EU sanctions against the Syrian regime. The bloc imposed a number of sanctions against the Assad regime for its brutal crackdown on civilians at the outset of the Syrian conflict in 2011.

U.S. national security adviser John Bolton applauded the seizure as “excellent news,” vowing that “America and our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade.”