A Royal Navy ship reportedly aimed its guns at Iranian boats after they attempted to seize a British oil tanker.  HMS Montrose was escorting the British Heritage tanker through the Persian Gulf when the stand-off with five Islamic Revolutionary Guard Corps boats took place on Wednesday, according to CNN.  It is claimed the boats told the tanker to change course and enter Iran's territorial waters, with a US aircraft recording the incident.  HMS Montrose, which was trailing the tanker, then trained its guns on the Iranian vessels and warned them to back off.  The Iranian boats obeyed the frigate's instructions, which were issued over the radio.  One US official told Reuters: "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."  Another official added: "It was harassment and an attempt to interfere with the passage."  Britain's Ministry of Defence had no immediate comment.  Thirty British commandos stormed an Iranian oil tanker bound for Syria last week, prompting Iran's President to threaten the UK with retaliatory action.  Speaking on state television on Wednesday, President Hassan Rouhani said: “You (Britain) are the initiator of insecurity and you will realise the consequences later.  "Now you are so hopeless that, when one of your tankers wants to move in the region, you have to bring your frigates (to escort it) because you are scared. Then why do you commit such acts (seizure)? You should instead allow navigation to be safe."  Iran was accused of attacks on six tankers either in or near the Strait of Hormuz in the last few months.  The US and Saudi Arabia, its major ally in the Middle East, blamed Iran for the incidents, although Tehran denied responsibility.  Tensions between Iran and the US increased last month when the Islamic Revolutionary Guard Corps shot down an American drone.