A British oil tanker was threatened by armed Iranian boats in the Persian Gulf on Wednesday, CNN reported, citing U.S. officials.

The Heritage tanker was crossing into the Strait of Hormuz when it spotted five armed Iranian Islamic Revolutionary Guard Corps boats trying to force it into Iranian waters.

The Iran boats backed away after a warning by the Royal Navy frigate HMS Montrose, which had been escorting the tanker.

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

Oil CLU19, +0.43% shot to the highest level since May on Wednesday amid fears of a hurricane in the Gulf of Mexico and signs of lower U.S. inventories.

Crude was holding just above those highs on Thursday.

CNN