Three Iranian ships attempted to "impede the passage" of a British oil tanker in Gulf waters, forcing a UK frigate to intervene, the British government said on Thursday.
"Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz," a British government statement said of the incident, which occurred on Wednesday.
"HMS Montrose was forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away. We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region," the statement said.
On Wednesday, Iran's President Hassan Rouhani had warned Britain of "consequences" over the detention of an Iranian oil tanker off the coast of Gibraltar.
"I point out to the British that you initiated insecurity (on the seas) and you shall grasp the consequences of it later on," Rouhani had said in comments to the cabinet broadcast by state TV.
Iran's president had called the seizure of the tanker "a foolish act."
The 330 metre (1,000 feet) Grace 1 tanker, capable of carrying two million barrels of oil, was halted on Thursday by police and customs in Gibraltar -- a tiny British overseas territory on Spain's southern tip - with the aid of a detachment of British Royal Marines.
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.
Iran denied it, saying that the destination "was somewhere else."
"The port named in Syria does not even have the capacity for such a supertanker to dock," Iran's deputy foreign minister Abbas Araghchi had said.
Iran's President Hassan Rouhani had warned Britain of "consequences" over the detention of an Iranian oil tanker off the coast of Gibraltar.