A senior commander of Iran’s Islamic Revolutionary Guard Corps said on Friday that if Britain does not release the vessel it detained in Gibraltar on Thursday on suspicion of transporting Iranian crude to Syria, it would be Iran’s “duty” to capture a British oil tanker.

The supertanker Grace 1 was seized by Gibraltarian authorities with assistance from British Royal Marines, on suspicion of bringing oil to Syria in violation of sanctions in place by the European Union. The ship is currently impounded off Gibraltar.

The Gibraltar government said in a statement that it had reasonable grounds to believe the ship was carrying crude oil to the Baniyas refinery in Syria.

IRGC Maj. Gen. Mohsen Rezaei, secretary of Iran’s Expediency Council, a powerful state body, wrote on Twitter: “If Britain does not release the Iranian oil tanker, it is the authorities’ duty to seize a British oil tanker.”

“Islamic Iran in its 40-year history has never initiated hostilities in any battles, but has also never hesitated in responding to bullies,” he wrote.

The seizure comes at a particularly delicate moment in relations between the United States and Iran, with the confrontation between the two countries over Iran’s nuclear program having assumed military dimensions in recent months.

A senior Iranian Foreign Ministry official accused the United Kingdom of seizing the ship “at the request of the United States.”

Spanish Foreign Minister Josep Borrell confirmed to reporters that the seizure of the Grace 1 had been prompted by a U.S. request to Britain. (Spain has a dispute with Britain over ownership of Gibraltar and believes the seizure took place in Spanish waters.)

U.S. National Security Advisor John Bolton tweeted in response to the news of the seizure: “Excellent news: U.K. has detained the supertanker Grace I laden with Iranian oil bound for Syria in violation of E.U. sanctions.”

Europe has banned oil shipments to Syria since 2011.