Tehran warned Tuesday that Britain’s seizure of an Iranian oil tanker in the waters off Gibraltar will not go “unanswered” and the country should “brace” for a robust response.

British Royal Marines boarded the tanker last Thursday under the suspicion it was transporting oil in violation of the European Union sanctions against Syria.

According to the semi-official Tasnim news agency, Iran’s armed forces chief of staff, Major General Mohammad Bagheri, said Iran would respond.

The Royal Navy was also told it had captured the supertanker in retaliation for the “direct, transparent and courageous” move by the “heroic Iranian Armed Forces” to shoot down a U.S. spy drone in June, he added.

“This measure (seizure of the oil tanker) will not go unanswered, and if necessary, the response to such a shameful move will be put on the agenda, at an appropriate time and place,” the general said.

He also alleged the downing of the American drone was testimony to Iran’s growing military prowess, adding, “Today, the Islamic Republic of Iran enjoys the necessary power in the missile, naval and air defense spheres, and this fact has posed a serious challenge to one of the enemy’s strong points, namely its aerial and missile capability.”

White House National Security Advisor John Bolton is on the record applauding 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, has also 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.