The troops killed two pirates, wounded one and arrested another four on one of the commercial ships, the official said. The tanker was on its way back to the southern port city of Aden from Al-Mohra province in eastern Yemen, where it had unloaded its cargo, the official said without giving details on the crew or vessel's tonnage.

The defence ministry's website, www.26sep.net, posted a report earlier on Sunday that two pirates were killed in the confrontation over the tanker, and three other pirates and two coast guards were wounded.

The Kenya-based environmental organisation Ecoterra International said on Sunday that Somali pirates had released a small Yemeni freighter MT Sea Princess II and its 15 crew members, held since January.

Some 16 other ships and more than 270 hostages are still held by Somali pirates pending the outcome of negotiations over ransom money for their release.

According to the International Maritime Bureau, pirate attacks off Somalia increased tenfold in the first three months of this year compared with the same period in 2008, jumping from six to 61.

Pirates off lawless Somalia - without an effective central government since 1991 - have defied an increased international naval presence to step up attacks during favourable weather, seizing more than 10 vessels in April alone.

The Bahrain-based US Fifth Fleet, a member of the international counter-piracy naval force Combined Task Force (CTF) 151, said it had no details on Sunday's attack. CTF 151 was established in January to fight piracy in the Gulf of Aden and the Indian Ocean.

Somali pirates attacked more than 130 merchant ships in the Gulf of Aden last year, an increase of more than 200 percent on 2007, according to the International Maritime Bureau, which tracks piracy.

Heavily armed pirates operate high-powered speed boats and sometimes hold ships for weeks before releasing them for large ransoms paid by governments or ship owners. More than 150 suspected pirates were arrested by naval patrols in the Gulf in 2008. -- AFP