Iranian gunships on Wednesday attempted to seize a British oil tanker traveling through the Strait of Hormuz in the Persian Gulf, but were foiled by a Royal Navy frigate, CNN reported.

According to the report, which cited sources in the US Pentagon, five gunships from the Islamic Revolutionary Guard Corps surrounded the British Heritage oil tanker and tried to force it off course into Iranian territorial waters.

However, a British warship, the HMS Montrose, which was escorting the tanker in the wake of increased tensions in the area, confronted the Iranian boats and forced them away.

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The Montrose trained its guns on the IRGC boats and issued a verbal warning, CNN said.

According to the report, a US aircraft overhead recorded the incident, but the US military has not yet released the video.

Iran’s President Hassan Rouhani had warned Britain of “consequences” on Wednesday over the detention of one of the Islamic Republic’s oil tankers 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 said in comments to the cabinet broadcast by state TV.

The detention of the 330-meter (1,000-feet) Grace 1 vessel comes at a sensitive time in Iran-EU ties as the bloc mulls how to respond to Tehran announcing it breached the uranium enrichment limit it agreed to in a troubled 2015 nuclear deal.

The Grace 1 tanker was halted in the early hours of Thursday by police and customs agencies in Gibraltar, aided by a detachment of British Royal Marines.

The ship was detained 2.5 miles (four kilometers) south of Gibraltar in what the UK considers British waters, although Spain, which lays claim to the territory, says they are Spanish. It was boarded when it slowed down in a designated area used by shipping agencies to ferry goods to vessels.

“We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria,” Gibraltar’s Chief Minister Fabian Picardo said in a statement.

“That refinery is the property of an entity that is subject to European Union sanctions against Syria.

Spanish Foreign Minister Josep Borrell told reporters the vessel was detained at the request of the United States.

In a statement, Britain’s Foreign Office said “we welcome this firm action by the Gibraltarian authorities, acting to enforce the EU Syria Sanctions regime.”

European Union sanctions against war-torn Syria have been in force since late 2011. The 28-member bloc has imposed sanctions on Syrian officials including government ministers over their role in the “violent repression” of civilians. It has frozen the assets of around 70 entities and introduced an embargo on Syrian oil, investment restrictions and a freeze on Syrian central bank assets within the EU.

The tanker’s detention comes just days after Iran announced it would exceed the uranium enrichment limit set up as part of the 2015 deal to avoid it building up to the level required for a nuclear warhead.

Tehran took the action in response to Washington abandoning the nuclear deal last year and hitting Iran’s crucial oil exports and financial transactions with biting sanctions.

The unilateral move has sent tensions in the Gulf soaring as the administration of US President Donald Trump forges ahead with a policy of “maximum pressure” against Iran in coordination with its Middle East allies Israel and Saudi Arabia.