Iran warned the UK government Saturday it should be “scared” of Tehran’s possible retaliation if it doesn’t release an Iranian oil supertanker siezed in the waters off Gibraltar.

British Royal Marines boarded the tanker Thursday under the suspicion it was transporting oil in violation of the European Union sanctions against Syria. The tanker’s crew were asked about the origin of the cargo and its destiniation. The crew consists of Indians, Pakistanis and Ukrainians. They have remained aboard the ship.

Authorities in Gibraltar, a British overseas territory on Spain’s southern tip at the western entrance to the Mediterranean, said they suspected the tanker was carrying crude to Syria in violation of E.U. sanctions that began in 2011.

White House National Security Advisor John Bolton applauded the UK’s intervention.

Excellent news: UK has detained the supertanker Grace I laden with Iranian oil bound for Syria in violation of EU sanctions. America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade. — John Bolton (@AmbJohnBolton) July 4, 2019

The detention of the 330-metre (1,000-feet) Grace 1 comes at a sensitive time in Iran-EU ties as the bloc mulls how to respond to Tehran announcing it is poised to breach the uranium enrichment limit it agreed to in a troubled 2015 nuclear deal.

Iran denies any wrongdoing. Instead, Mohammad Ali Mousavi Jazayeri, a member of the Assembly of Experts, a powerful religious body, warned the UK government directly of consequences for its actions. He said:

I am openly saying that Britain should be scared of Iran’s retaliatory measures over the illegal seizure of the Iranian oil tanker. We have shown that we will never remain silent against bullying … As we have a staunch response to the American drone, the appropriate response to the illegal capture [of the tanker] will be given by Iran as well.

Expediency Discernment Council Secretary Mohsen Rezaee took it a step further when he tweeted “if the UK does not release the Iranian oil tanker, our officials are duty-bound to reciprocate and seize a British oil tanker.”

Rezaee previously served as the commander of the Iranian Revolutionary Guard Corps. but now he’s an adviser to Iran’s Supreme Leader.

UPI contributed to this story