The Indian navy today defended its sinking of a ship in the Gulf of Aden despite official confirmation that it was not a pirate vessel but a hijacked Thai fishing boat.

The International Maritime Bureau said that what the Indian navy described as a pirate "mothership" was the Ekawat Nava 5, which had been seized by pirates off Yemen on November 18. Its crew were kidnapped at gunpoint and tied up.

India's navy said last week that the Tabar, on patrol in the Gulf of Aden, had blown up one vessel and chased off two smaller pirate speedboats. It was the first major offensive by a country protecting shipping lanes across 2.6m sq km of ocean.

Apparent inconsistencies in the story emerged when Bangkok-based Sirichai Fisheries, which owned the trawler, told maritime officials there had been a mistake.

Wicharn Sirichaiekawat, the owner of the ship, said the Ekawat Nava 5 had left Oman for Yemen to deliver fishing equipment and had been taken by Somali pirates. "The sunken ship, which the Indian navy claimed was a mothership of pirates, was not the mothership at all," he said. The only surviving sailor in the crew of 15, a Cambodian who spent six days adrift, is recovering in a hospital in Yemen. There is no word on any of the other crew. The Indian navy said the ship was a pirate vessel in "description and intent" and had opened fire first. A navy spokesman, Commander Nirad Sinha, said the Tabar had taken action only because pirates on board the ship threatened to attack. "We fired only after we had been threatened and then fired upon," he said. "This is self-defence. Our ship was attacked by rocket-propelled grenades."

Naval sources said the Indian frigate hit the Ekawat Nava 5 several times and set off explosions.

"A fire broke out, probably due to the exploding ammunition," an officer from the Tabar said. Two speedboats were spotted breaking off to escape. If it was just another fishing ship why would it have men with rockets on board?"

Other sources said the Tabar had successfully escorted around 35 ships.

Somalia's lawless shoreline has made it a haven for pirates, who have been roaming further and further out to sea and attacking bigger ships.

There have been 96 pirate attacks this year in Somali waters, resulting in 39 ships being hijacked. More than a dozen ships with almost 300 crew are still in the hands of pirates, who are demanding millions of dollars in ransom.

Experts said the Indian navy's increasingly muscular approach to the problem could not be criticised. A third of India's 900 cargo ships are considered at risk from piracy. "The route through the Gulf of Aden is a link to the Suez Canal; it is a vital link especially for India's energy needs. So they are trying to do something to protect their own interests and shipping," said Nimal Perera of Aitken Spence, a shipping agency in Colombo. Perera said the root of the problem – the pirate bases in Somalia – had to be dealt with.