TEHRAN: Iran's Revolutionary Guards said Thursday that the United States and Britain will "strongly regret" the seizure of a tanker off Gibraltar, the semi-official Fars News Agency reported.

"If the enemy had made the smallest assessment they wouldn't have done this act," said Rear-Admiral Ali Fadavi, deputy commander in chief of the Revolutionary Guards, referring to the seizure of an oil tanker late last week by Gibraltar's police aided by British Royal Marines.

He added that the seizure of the tanker was "stupidity... a trait the American President has in spades and the British to some extent."

The Guards also denied on Thursday that they had in turn impeded a British oil tanker in the Strait of Hormuz in a statement published by the force's Sepahnews site.

"There has been no confrontation in the last 24 hours with any foreign vessels, including British ones," the statement said.

Britain's government said Thursday three Iranian boats had attempted to "impede the passage" of a British oil tanker in Gulf waters, forcing UK warship HMS Montrose to intervene.

The Guards' statement said that if they were ordered to seize foreign vessels they would do so "immediately, decisively and speedily."

Iran's Foreign Minister Mohammad Javad Zarif called reports alleging that the Guards tried to impede the British oil tanker "claims to create tension (in the region,) these claims are worthless," the official state news agency IRNA reported.

Referring to last week's seizure of a tanker off Gibraltar, Fadavi said that the vessel had "been leased" to "transfer cargo."

Iran condemned the detention as an "illegal interception," but Gibraltar officials said that the cargo was believed to be destined for Syria, which is subject to European sanctions.

The 330 metre (1,000 feet) Grace 1 tanker, capable of carrying two million barrels of oil, was halted on July 4.

Iranian President Hassan Rouhani warned Britain on Wednesday of the "consequences" of what he described as "a foolish act".

"I point out to the British that you initiated insecurity (on the seas) and you shall grasp the consequences of it later on," Rouhani said in comments to the cabinet broadcast by state TV.

On Monday Iran's defence minister had vowed to respond to Britain's move, which he called an act of maritime piracy.

Brigadier-General Amir Hatami said it "will not be tolerated by us."

Iranian-US tensions have spiked in recent weeks, with Washington blaming Tehran for multiple attacks on ships, and the Islamic republic shooting down an American surveillance drone. —AFP