Two Indian vessels have had to reverse course in the Strait of Hormuz following reports of gunfire from Iran's Revolutionary Guard, a vessel-tracker said.

TankerTrackers.com said the vessels include an Indian-flagged super tanker, carrying 2 million barrels of Iraqi oil.

Earlier, the British military said two gunboats from Iran's Revolutionary Guard opened fire on a tanker transiting the Strait of Hormuz after Iran said it had reimposed restrictions on the vital waterway.

The United Kingdom Maritime Trade Operations Centre said the tanker and crew were reported safe, without identifying the vessel or its destination.

Iran said earlier it was reimposing restrictions on the strait in response to a US blockade on Iranian shipping and ports. Iran has prevented vessels from crossing throughout the seven-week-long war, except for ones it authorises.

Merchant vessels attempting to cross the Strait of Hormuz also received radio messages from Iran's navy telling them they were not allowed to pass, while at least two ships reported being hit by gunfire, shipping sources said.

Several commercial vessels tried to transit the strait after receiving a notice to mariners a day earlier saying passage would be allowed but restricted to lanes Iran deemed safe.

The incidents were reported in waters between the Qeshm and Larak islands, with vessels turning back without completing the crossing, sources said.

A container ship was also hit by gunfire, a maritime security source said.

Some vessels reported that Iran's navy had been broadcasting a VHF message declaring the Strait of Hormuz closed again and that no vessel of any type or nationality was allowed to pass.

Hundreds of ships and about 20,000 seafarers remain stranded in the Gulf, waiting to pass through the key waterway, which handles about 20 per cent of global oil and liquefied natural gas flows.

In a separate incident, UKMTO said it had received a report of a container ship being hit by an unknown projectile 25 nautical miles northeast of Oman.

The attack caused damage to some containers on board, but no fires or environmental impact were reported, and there were no immediate reports of casualties, UKMTO said.