NEW DELHI — An Indian naval vessel sank a suspected pirate "mother ship" in the Gulf of Aden and chased two attack boats into the night, officials said today, as separate bands of brigands seized Thai and Iranian ships in the lawless seas.

The owners of a seized Saudi oil supertanker, meanwhile, negotiated for the release of the ship, anchored off the coast of Somalia.

A multinational naval force has increased patrols in the waters between the Arabian Peninsula and the Horn of Africa, where pirates have grown bolder and more violent. The force scored a rare success Tuesday when the Indian warship, operating off the coast of Oman, stopped a ship similar to a pirate vessel described in numerous bulletins. The Indian navy said the pirates fired on the INS Tabar after the officers asked to search it.

"Pirates were seen roaming on the upper deck of this vessel with guns and rocket propelled grenade launchers," said a statement from the Indian navy. Indian forces fired back, sparking fires and a series of onboard blasts — possibly due to exploding ammunition — and destroying the ship.

They chased one of two speedboats shadowing the larger ship. One was later found abandoned. The other escaped, according to the statement.

Larger "mother ships" are often used to take gangs of pirates and smaller attack boats into deep water, and can be used as mobile bases to attack merchant vessels.

Last week, Indian navy commandos operating from a warship foiled a pirate attempt to hijack a ship in the Gulf of Aden. The navy said an armed helicopter with marine commandos prevented the pirates from boarding and hijacking the Indian merchant vessel.

Separate bands of pirates also seized a Thai ship with 16 crew members and an Iranian cargo vessel with a crew of 25 in the Gulf of Aden, where Somalia-based pirates appear to be attacking ships at will, said Noel Choong of the International Maritime Bureau's piracy reporting center in Malaysia.

"It's getting out of control," Choong said.

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

"The criminal activities are flourishing because the risks are low and the rewards are extremely high," Choong said.

The pirates used to mainly roam the waters off the Somali coast, but now they have spread in every direction and are targeting ships farther at sea, according to Choong.

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 supertanker carrying $100 million in crude.

The supertanker, the MV Sirius Star, was anchored Tuesday close to Harardhere, the main pirates' den on the Somali coast, with a full load of 2 million barrels of oil and 25 crew members.

Asked about reports that a ransom had been demanded, Saudi Foreign Minister Prince Saud al-Faisal said Wednesday that the owners of the tanker "are negotiating on the issue." He did not elaborate.

He said "we do not like to negotiate with pirates, terrorists or hijackers." But he said the owners of the tanker are "the final arbiter" on the issue.

Saudi Arabia, the world's leading oil producer, has condemned the hijacking and said it will join the international fight against piracy.

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. 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.

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

"Given the size of the area and given the fact that we do not have naval assets — either ships or airplanes — 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, which was flying a flag from the tiny Pacific nation of Kiribati but operated 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.

Wicharn Sirichaiekawat, manager of Sirichai Fisheries Co., Ltd. told The Associated Press that the ship, the "Ekawat Nava 5," was headed from Oman to Yemen to deliver fishing equipment.

"We have not heard from them since so we don't know what the demands are," Wicharn said. "We have informed the families of the crew but right now, we don't have much more information to give them either."

Later in the day, Thai Foreign Ministry deputy spokesman Voradet Veeravekin told The Associated Press that Thai officials in Kenya were trying to make contact with the vessel.

"Based on previous cases, we believe they were held for ransom. We are optimistic that we will be able to negotiate for their release once we can contact the ship," he said.

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

"We will no longer expose our crew to the risk of being hijacked and held for ransom by pirates in the Gulf of Aden," said Terje Storeng, Odfjell's president and chief executive.