The UK will deploy a second warship to the Persian Gulf after three Iranian vessels attempted to impede the passage of a British tanker transiting through the Strait of Hormuz, the only passage from the Persian Gulf to the open ocean.

The Type 45 frigate HMS Duncan will sail to the Gulf within the next week to join HMS Montrose, after it has completed a NATO exercise in the Black Sea.

The incident at the Strait of Hormuz occurred amid mounting tensions between Iran and the UK after British Royal Marines stormed and impounded an Iranian supertanker off Gibraltar last week over suspicions of attempting to violate international sanctions on Syria.  Iran had threatened to seize a British ship in retaliation.

The attempt to block and divert the oil tanker British Heritage was reported a day after Iran President Hassan Rouhani warned Britain of repercussions over the seizure of the Iranian tanker.

“You are an initiator of insecurity and you will understand its repercussions,” Rouhani warned London. 

According to the British Ministry of Defense, the Royal Navy warship HMS Montrose positioned herself between the Iranian gunboats and the British Heritage. The Iranian vessels turned away after the crew of the Montrose issued verbal warnings and trained deck guns at them.

"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region,” the Ministry of Defense said in a statement. 

Iran, however, denied the allegations. 

“Apparently the British tanker has passed. What [the British] have said themselves and the claims that have been made are for creating tension and these claims have no value,” said Iran foreign minister Javad Zarif.

The British Heritage is owned by U.K. petrochemical giant BP. The Century Class oil tanker is one of the largest of BP’s crude oil tankers, which can carry up to a million barrels of oil at a time. 

A spokesperson for Prime Minister Theresa May said that the UK and the U.S. are discussing about increasing their military presence in the Gulf after the latest incident.

The Ministry of Defence did not comment on the deployment of the second warship but Duncan was always expected to travel to the region to relieve HMS Montrose when it comes off duty for repair and maintenance.