GIBRALTAR — Iran on Thursday condemned the "illegal interception" of one of its oil tankers in Gibraltar where authorities said they suspected it was carrying crude to Syria in violation of EU sanctions.

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

Iran summoned the British ambassador in protest.

"After the illegal interception of an Iranian oil tanker in the strait of Gibraltar by the English naval force, that country's ambassador to Tehran was summoned to the foreign ministry," the ministry's spokesman Abbas Moussavi said in Tehran.

However, White House National Security Advisor John Bolton applauded the interception of the supertanker.

"Excellent news: UK has detained the supertanker Grace I laden with Iranian oil bound for Syria in violation of EU sanctions," Bolton tweeted. 

The Grace 1 tanker was halted in the early hours of Thursday by police and customs agencies in Gibraltar, a tiny overseas British territory on Spain's southern tip.

They were aided by a detachment of British Royal Marines. — AFP