British Royal Marines boarded an Iranian tanker, Grace 1, off Gibraltar last week and seized it. (File)

Britain and the United States will regret detaining an Iranian oil tanker, a deputy commander of Iran's elite Revolutionary Guards said on Thursday, according to the Tasnim news agency.

Last week, British Royal Marines boarded an Iranian tanker, the Grace 1, off Gibraltar and seized it on suspicion that it was breaking EU sanctions by taking oil to Syria.

"Now an action that does not need ability but some stupidity has been carried out by them," Rear Admiral Ali Fadavi, a deputy Guards commander, said, according to Tasnim.

"The American government ... and also England .... should not have taken action if they had made the smallest calculation," he said.

"We had rented this ship and we carried the cargo. Their action was very silly and they will certainly regret it. Our reciprocal action will be announced."