Sana'a, Yemen  - Somali pirates opened fire at a Japanese oil tanker off Yemen's territorial waters in the Gulf of Aden on Monday, but no reports of casualties among crew, a Yemeni port official said.

The official at the southern Yemeni port of Aden told Deutsche Presse-Agentur dpa that the ship was likely hit by a rocket fired by pirates sailing on a small ship.

A source at the Japanese embassy in Sana'a said the ship was slightly damaged and would continue its trip to the Saudi port of Jeddah on the Red Sea.

The source, who spoke on condition of anonymity, said the tanker was six hours from the Yemeni port of Aden when it was attacked.

No further details were immediately available.

Piracy off the Somali coast - which lies at the mouth of the Red Sea - has been rife since the country slid into chaos after dictator Mohamed Siad Barre was ousted by warlords in 1991.

Frequent pirate attacks in the Gulf of Aden, a major trade route between Asia and Europe, has made those waters the world's most dangerous for pirate activities.

The International Maritime Bureau has advised vessels that are not making scheduled calls to ports in Somalia to keep a distance of 200 nautical miles away from the Somali coast. (dpa)