Iranian forces tried to stop and divert a British oil tanker as it crossed into the Strait of Hormuz on Wednesday, the British government said in a statement.

"Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz," it said.

The Iranian vessels only turned away after the UK naval frigate HMS Montrose intervened, positioning itself between the ships and the British tanker. During the maneuver, it urged the Iranian vessels to leave the area in order to avoid any further action.

'We need to show confidence'

British Foreign Minister Jeremy Hunt, who is vying for the prime minister's office, said the incident was a reminder that the UK "must invest more in defense."

"We need to show confidence in the world by putting our money where our mouth is," Hunt said in a tweet.

Read more: Tanker attacks in the Gulf — evidence or warmongering?

'No incident'

Meanwhile, the Iranian Revolutionary Guard, which patrols the waters, denied that any incident had taken place. "In the last 24 hours, we had no incident involving foreign vessels in the Strait of Hormuz, also not with a British one," a statement said.

The vessel, which was leaving the Persian Gulf at the time of the reported incident, was on Thursday seen to have passed the Strait of Hormuz and was passing through the Gulf of Oman.

Russia urged restraint, saying that it was aware of both Britain's statement on the incident and Iran's denial. Kremlin spokesman Dmitry Peskov said "freedom of navigation should be ensured in the Persian Gulf and the Strait of Hormuz."

Read more: Iran and Saudi Arabia need a 'fresh security architecture'

Warning of consequences

Iranian President Hassan Rouhani on Wednesday had warned Britain of unspecified "consequences" over the detention of an Iranian oil tanker last week off the British overseas territory of Gibraltar. But shortly after Thursday's incident, Gibraltar police arrested the tanker's captain for violating EU sanctions.

The Grace 1 tanker was halted by British marines and local police on its way from Iran to Syria, via the Mediterranean.

While officials in Gibraltar said they believed the cargo was destined for Syria, which is subject to EU sanctions, Iran condemned the detention as an "illegal interception."

Tensions around the Strait of Hormuz have heightened significantly since the US deployed military assets to the area and stepped up economic sanctions against Tehran.

Under a plan announced on Tuesday, the US has called on allies to join it in a coalition that would patrol waters off Yemen.

The US would provide "maritime domain awareness and surveillance," while each coalition member would provide protection to the commercial vessels that fly under its flag.

Read more: Iran-US conflict: Tehran's asymmetrical approach

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ls, rc/msh (dpa, AP, Reuters)