"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," a British government spokesman said in a statement.

The incident was said to be filmed by a US aircraft overhead.

The US officials, speaking on condition of anonymity, said the incident happened as the vessel British Heritage was at the northern entrance of the Strait of Hormuz.

"The Royal Navy HMS Montrose, which was also there, pointed its guns at the boats and warned them over radio, at which point they dispersed," one of the officials said.

"It was harassment and an attempt to interfere with the passage," the other official said.

The incident came almost a week after 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.

The move sparked a diplomatic row between Britain and Iran, with Tehran summoning the British ambassador.

Iran's armed forces chief of staff, Major General Mohammad Bagheri, had said the British seizure would not go unanswered but the Islamic Republic denied it had sought to stop the British Heritage.

Earlier on Wednesday, Iranian President Hassan Rouhani said Britain would face "consequences" over the seizure of the Iranian tanker.

Washington has accused Tehran of being behind a series of recent attacks against commercial vessels near the Strait of Hormuz.

Last month, Trump said he came close to ordering an air strike in retaliation against Iran for shooting down a US spy drone near the Strait.

General Joseph Dunford, chairman of the US military's Joint Chiefs of Staff, said on Tuesday the US was attempting to build a military coalition to protect shipping in the region.

"We are engaging now with a number of countries to see if we can put together a coalition that would ensure freedom of navigation both in the Straits of Hormuz and the Bab el Mandab," Dunford said.

DPA, Reuters