The Indian navy said the pirates fired on the INS Tabar, it fired back, destroying the ship, and then chased another speedboat, later found abandoned.

Meanwhile in the area separate bands of pirates hijacked two ships and captured their crews, clear signs that the gangs roaming the Gulf are becoming bolder and more violent.

A Thai ship with 16 crew members and an Iranian cargo vessel with a crew of 25 were hijacked on Tuesday, said Noel Choong of the International Maritime Bureau's piracy reporting centre in Malaysia, adding: "It's getting out of control."

The incidents raised to eight the number of ships hijacked this week alone. Since the beginning of the year 39 ships have been hijacked in the Gulf of Aden, out of 95 attacked.

"There is no firm deterrent, that's why the pirate attacks are continuing," Mr Choong said.

He said 17 vessels remain in the hands of pirates along with more than 300 crew members, including a Ukrainian ship loaded with weapons and the Saudi Arabian supertanker Sirius Star carrying two million barrels of oil.

Despite increased patrols by Nato and other forces, the attacks have continued unabated off Somalia, which is caught up in an Islamic insurgency and has had no functioning government since 1991. Pirates have generally released ships they have seized after ransoms are paid.

Nato has three warships in the Gulf of Aden and the US Navy's Bahrain-based 5th Fleet also has ships in the region.

But Commander Jane Campbell of the 5th Fleet said naval patrols cannot prevent attacks given the vastness of the sea and the 21,000 vessels going through the Gulf of Aden every year.

"Given the size of the area and given the fact that we do not have naval assets... to be everywhere with every single ship" it would be virtually impossible to prevent every attack, she said.

The Gulf of Aden connects to the Red Sea, which in turn is linked to the Mediterranean by the Suez Canal. The route is thousands of miles and many days shorter than going around the Cape of Good Hope off the southern tip of Africa.

The Thai boat, flying a flag from the tiny Pacific nation of Kiribati but operating out of Thailand, made a distress call as it was being chased by pirates in two speedboats but the phone connection was cut off midway.

The Iranian carrier was flying a Hong Kong flag but operated by the Islamic Republic of Iran Shipping Lines.

On Tuesday major Norwegian shipping group Odfjell ordered its more than 90 tankers to sail around Africa rather than use the Suez Canal after the seizure of the Saudi tanker on Saturday.

Meanwhile the families of two British crewmen being held hostage by pirates on hijacked oil tanker Sirius Star said yesterday they hope the pair "will be home safely very soon".

Peter French, from County Durham, and James Grady, from Strathclyde, are on the ship, which is currently anchored off the coast of Somalia.

A statement said the families of the two men "greatly appreciate the concern that has been expressed by people throughout the UK and beyond. They hope Peter and James will be home safely to their families very soon."

The men are the tanker's chief engineer and its Second Officer, although the Foreign Office could not confirm which man held which role.

The Sirius Star was attacked at the weekend and the pirates who have taken control of the vessel have taken it to a stronghold near the town of Eyl.

It is the largest vessel ever to be hijacked in a region which has become notorious for piracy.

Foreign Secretary David Miliband has called for the immediate release of the crew.

He said the problem of piracy was "a grave danger to the stability in the region".

Speaking as he arrived in Beirut for talks with the Lebanese government, Mr Miliband said: "We call on those holding the Britons to release them and the rest of the crew immediately.

"The Royal Navy is co-ordinating the European response as well as contributing to the international mission there.

The vessel's operator, Dubai-based Vela International Marine, said the 25 crew on board – which also includes two Poles, one Croatian, one Saudi national and 19 Filipinos – were believed to be safe.