Unknown assailants in a speedboat shot at a crude oil tanker as it sailed through the Strait of Hormuz between Iran and Oman on Sunday, the International Maritime Bureau (IMB) said.

The crew of the unidentified oil tanker reported being shot at twice from close range from a speedboat carrying six people with machine guns on Sunday morning. They repelled the attack with hoses and the tanker and crew are safe.

Although suspected Somali pirates commonly target vessels in the Gulf of Aden and Somali Basin, attacks on shipping in the Strait of Hormuz - a vital oil and gas shipping route - are rare.

The shooting happened on the Gulf of Oman side of the Strait of Hormuz, about 90 minutes after an unidentified merchant ship was approached by two speedboats with crews wearing military clothing, the NATO Shipping Centre (NSC) said on Sunday.

"Two green-coloured skiffs with three-four persons on board in military clothing and armed with gun machines got to 150 metres of a merchant vessel," NATO's merchant shipping alert service said in a statement.

"After awhile the skiffs turned away to Iranian coast."

No shots were fired by the crews wearing military uniforms who approached the unidentified merchant ship. That incident happened about 30 nautical miles to the west of the oil tanker shooting committed by a different speedboat.

A spokesman for the NATO Shipping Centre said the two incidents were being investigated. He said the nationalities of the non-merchant crews had not yet been determined and that it was too early to say whether the two incidents were related.-Reuters