The Indian navy is now patrolling off the Somali coast
An Indian navy warship has destroyed a suspected Somali pirate vessel after it came under attack in the Gulf of Aden.
INS Tabar sank the pirate "mother ship" after it failed to stop for investigation and opened fire instead, an Indian navy statement said.
There has been a surge in piracy incidents off the coast of Somalia.
The latest attack came days after the Saudi-owned Sirius Star supertanker and its 25 crew were seized by pirates and anchored off the Somali coast.
Vela International, operators of the Sirius Star, told the BBC no demands had yet been received from the pirates. The company also said all the crew were safe.
The biggest tanker ever hijacked, Sirius Star is carrying a cargo of two million barrels of oil - a quarter of Saudi Arabia's daily output - worth more than $100m (£67m).
Indian Navy spokesman Commander Nirad Sinha describes the attack
Analysts say the pattern of other hijackings suggests a ransom request is likely to follow. Given the value of the tanker and its cargo, that is expected to be a sizeable demand.
Two of the captive crew are British. The UK Foreign Secretary, David Miliband, said the Royal Navy was co-ordinating the European response to the incident.
"The problem of piracy around Somalia is a grave danger to the stability in the region," he told the BBC.
Somalia has not had a functioning national government since 1991 and has suffered continuing civil strife.
India is among several countries already patrolling the Gulf of Aden, one of the world's busiest shipping lanes which connects the Red Sea and the Indian Ocean.
The Indian navy said the Tabar spotted the pirate vessel while patrolling 285 nautical miles (528km) south-west of Salalah in Oman on Tuesday evening.
The navy said the pirates on board were armed with guns and rocket propelled grenade launchers.
When it demanded the vessel stop for investigation, the pirate ship responded by threatening to "blow up the naval warship if it closed on her", the statement said.
Pirates then fired on the Tabar, and the Indians say they retaliated and that there was an explosion on the pirate vessel, which sank.
"Fire broke out on the vessel and explosions were heard, possibly due to exploding ammunition that was stored in the vessel," the Indian navy said.
Some of the pirates tried to escape on two speedboats. The Indian sailors gave chase but one boat was later found abandoned, while a second boat escaped.
INS Tabar has been patrolling the Gulf of Aden since 23 October, and has escorted 35 ships safely through the "pirate-infested waters", the statement said.
Last week, helicopter-borne Indian marine commandos stopped pirates from boarding and hijacking an Indian merchant vessel.
On Tuesday, a cargo ship and a fishing vessel became the latest to join more than 90 vessels attacked by the pirates this year.
THE SIRIUS STAR
Carrying 2m barrels of oil
Biggest vessel to be hijacked
The first vessel, a 25-crew cargo vessel transporting wheat to Iran, was attacked in the Gulf of Aden, while contact was lost with the crew of 12 on the fishing boat.
Piracy off the coast of East Africa and the Gulf of Aden - an area of more than 1m sq miles (2.6m sq km) - is estimated to have cost up to $30m in ransoms this year, a UK think tank has said.
The hijackings account for one-third of all global piracy incidents this year and the situation is getting out of control, according to the International Maritime Bureau.
The pirates who seized the Sirius Star are a sophisticated group with contacts in Dubai and neighbouring countries, says the BBC Somali Service's Yusuf Garaad.
Much of their ransom money from previous hijackings has been used to buy new boats and weapons as well as develop a network across the Horn of Africa, he adds.
Shipping companies are now weighing up the risks of using the short-cut route to Europe via the Suez canal.
However, travelling around South Africa's Cape of Good Hope would add several weeks to average journey times and substantially increase the cost of goods for consumers.