Separate bands of pirates hijacked two ships and captured their crews, while yet another opened fire on an Indian navy ship before being driven off - clear signs that the pirates roaming the Gulf of Aden are becoming bolder and more violent, officials said on Wednesday.

The Indian Navy confirmed that their warship the INS Tabar, which is designated to fight pirates, destroyed a suspected pirate ship that had opened fire in the Gulf of Aden late on Tuesday.

Speaking from the Ministry of Defence in New Delhi on Wednesday, Nirad Sinha, press officer for the Indian Navy said the INS Tabar had approached the pirate ship and asked it to stop to be searched.

The pirate ship, which appeared to be a "mother vessel" loaded with food, diesel and water, had two speed boats in tow.

Sinha said that naval officers could see men roaming the ship's deck with rocket propelled grenade launchers and guns.

He said the pirates threatened to blow up the warship, and then opened fire.

The INS Tabar returned fire, sparking explosions and a fire and destroying the suspected "mother vessel" pirate ship.

The Indian ship chased one of the speed boats, which was later found abandoned, while the other escaped, the Indian Navy said.

This would be the third attack the INS Tabar has fended off since it began its anti-piracy mission in the Gulf of Aden on November 2.

In an earlier incident, naval commandos flying in a helicopter that was launched from the Tabar foiled an attempt to hijack Indian merchant vessel Jag Arnav on November 11.

The attack took place 285 nautical miles southwest off Salalah, Oman.

The Indian navy say they have been conducting anti-piracy patrols in the Gulf of Aden since 23 October in coordination with the ministry of shipping.

They say the patrols are intended to protect Indian merchant vessels from piracy and to instil confidence in their seafaring community.

Pirate attacks off the Somali coast have surged 75 percent this year, as bandits lured by million-dollar ransoms have pushed farther out to sea in search of bigger prey among the 20-thousand oil tankers, freighters and merchant vessels transiting the Gulf of Aden each year.

Despite the stepped-up patrols, the attacks have continued unabated off Somalia, which is caught up in an Islamic insurgency and has had no functioning government since 1991.

Managing Director of Hong Kong Shipowners Association Arthur Bowring said there is very little effective enforcement of policing.

It's "incredibly bad. It's lawless on the coast to shore," he told AP Television.

Without police action and control "the pirates gangs can really operate with almost total impunity," he said.

A Thai ship with 16 crew members and an Iranian cargo vessel with a crew of 25 were also hijacked on Tuesday in the Gulf of Aden according to the International Maritime Bureau's piracy reporting centre in Malaysia.

According to the International Maritime Bureau 17 vessels remain in the hands of pirates along with more than 300 crew members, including a Ukrainian ship loaded with weapons.

In their most audacious hijacking to date, Somali pirates on Saturday seized the Saudi supertanker, the MV Sirius Star, which is carrying a full load of 2 million barrels of oil worth a reported 100 million US dollars and 25 crew members.

Pirates have generally released ships they have seized after ransoms are paid.

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

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