An Indian warship sank a Thai trawler in the Gulf of Aden last week after wrongly assuming  it to be a Somali pirate "mother ship", an international maritime agency has said.

The International Maritime Bureau (IMB) said the trawler had just been taken over by pirates when the navy attacked, a move that was initially hailed as a blow to Somali pirates preying on the region's shipping lanes.

India's defence ministry, which earlier released pictures of the ship on fire after the incident, has admitted that it was a Thai fishing boat but said it fired in self-defence.

One Thai crew member is confirmed to have died when the Indian frigate, INS Tabar, fired on the boat on November 18. Fourteen others are still missing.

A Cambodian sailor from the trawler, which was headed from Oman to Yemen to deliver fishing equipment when it was hijacked, was rescued four days later by passing fishermen.

Noel Choong, who heads the IMB's piracy reporting center in Kuala Lumpur, said the IMB received a report on the mistake late on Tuesday from Bangkok-based Sirichai Fisheries, which owned the Ekawat Nava5 vessel.

"The Indian navy assumed it was a pirate vessel because they may have seen armed pirates on board the boat which has been hijacked earlier," said Choong.

"We are saddened with what has happened. We hope that this incident won't affect the anti-piracy operation by the multi-coalition navies there."

In New Delhi, Commander Nirad Sinha, an Indian navy spokesman, said the ship had apparently been hijacked by pirates and that the frigate was responding to the threat to attack.

The Indian warship was part of an international sea patrol in the Gulf of Aden [AFP]

"Only after we were fired upon did we fire. We fired in self-defense. There were gun-toting guys with RPGs [rocket-propelled grenade launchers] on it."

There have been 96 pirate attacks so far this year in Somali waters, and 15 ships with nearly 300 crew members were being held hostage by pirates who are demanding millions of dollars in ransom.

An international naval force deployed to patrol the seas of Somalia and the Gulf of Aden has been escorting some merchant ships and responding to distress calls in the area.

Shipping firms have also called for aggressive military approach to tackle piracy off Somalia's coast and prevent costly re-routing of vessels.