Armed Iranian boats unsuccessfully tried to seize a British oil tanker in the Persian Gulf Wednesday, according to two US officials with direct knowledge of the incident.

The British Heritage tanker was sailing out of the Persian Gulf and was crossing into the Strait of Hormuz area when it was approached by boats from the Iranian Islamic Revolutionary Guard Corps.

The Iranians ordered the tanker to change course and stop in nearby Iranian territorial waters, according to the officials. A US aircraft was overhead and recorded video of the incident, though CNN has not seen the footage.

A British Ministry of Defense source told CNN that it “appeared that the Iranian vessels were trying to divert the Heritage from international to Iranian waters” before the British Navy ship HMS Montrose “got between them and issued a verbal warning to withdraw.”

US officials said five Iranian ships approached the UK vessels. However, British officials said that three Iranian vessels were involved in the incident.

The UK’s Royal Navy frigate HMS Montrose had been escorting the tanker from the rear. It trained its deck guns on the Iranians and gave them a verbal warning to back away, which they did, according to the UK source. Montrose is equipped on the deck with 30 mm guns specifically designed to drive off small boats. UK officials had previously confirmed that the Montrose was in the region performing a “maritime security role.”

In a statement, the UK’s Ministry of Defense said that “contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz.”

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

Iran’s Revolutionary Guard denied that it tried to stop the British tanker, the semi-official Fars news agency reported Thursday.

On Thursday, the British Heritage was seen moving through the Gulf of Oman along the Omani coastline, according to tracking platform Marine Traffic.

The British Heritage is a British oil tanker operated by British Petroleum. It was not carrying any cargo at the time of the incident.

The vessel left the Iraqi city of Basra on July 6 and stopped off the coast of Saudi Arabia on July 7, according to the tracker.
On July 10, the ship turned off its transponders for almost 24 hours, making it undetectable by radars. When it switched on its transponders at around 1pm Eastern Time, it appeared to have sailed through the Persian Gulf escorted by the HMS Montrose.

The incident is yet another flashpoint in a series of maritime episodes involving Iran, coming less than a week after British Royal Marines stormed an Iranian ship that was suspected of carrying oil to Syria. Just last month, tensions between the US and Iran escalated into a military standoff after an American drone was shot down by Iran over the Straight of Hormuz.