Britain has announced that Iranian vessels tried to impede a British oil tanker in the Strait of Hormuz, but were turned away by a Royal Navy frigate escorting it.
The British Ministry of Defence called the Iranian action contrary to international law, and urged Tehran to de-escalate the situation.
A ministry statement says three boats approached the tanker and tried to stop it on Wednesday. The boats reportedly backed off after the British frigate moved between them and the tanker, issuing verbal warnings.
CNN and other US media earlier quoted US government officials as saying small boats of Iran's Revolutionary Guard tried to seize the tanker.
Iran's Foreign Minister Mohammad Javad Zarif told local media on Thursday that Britain made the claims to create tension in the region and that they have no value. He did not say whether Iran was involved.
The Fars news agency, which has ties to Iran's government, said the Revolutionary Guard denied trying to stop the tanker or having contact with any foreign ships in the last 24 hours.
The incident in the key oil shipping lane has increased concerns that tension in the Middle East may intensify further.
Earlier this month, authorities in the British territory of Gibraltar captured an Iranian supertanker, prompting a former Revolutionary Guard commander to threaten retaliatory action.