Unknown assailants in a speedboat shot at a merchant vessel as it sailed through the Strait of Hormuz between Iran and Oman on Sunday, the NATO Shipping Centre (NSC) said.

The unidentified merchant ship reported being shot at twice from close range from a speedboat carrying six people armed with machine guns, on Sunday morning. It repelled the attack with hoses and the vessel and crew are safe, NSC said.

Although suspected Somali pirates commonly attack merchant shipping 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 attack happened on the Gulf of Oman side of the Strait of Hormuz, about 90 minutes after a different merchant ship was approached by two speedboats with crews wearing military clothing, NATO said.

"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," the North Atlantic Treaty Organization's merchant shipping alert service said in a statement.

"After a while the skiffs turned away to Iranian coast."

No shots were fired by the crews wearing military uniforms. That incident happened about 30 nautical miles to the west of the shooting 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 to early to say whether the two incidents were related.-Reuters