NEW DELHI: An Indian naval vessel sank a pirate “mother ship” in the Gulf of Aden and chased two attack boats late on Tuesday, officials said yesterday as more violence took place in the lawless seas where brigands are becoming bolder and more violent.

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.”

A multicoalition naval force has increased patrols in the region and scored a rare success Tuesday when the Indian warship, operating off the coast of Oman, stopped a ship similar to a pirate vessel mentioned in numerous piracy bulletins. The Indian Navy said the pirates fired on the INS Tabar after the officers asked it to stop to be searched.

“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 that had been shadowing the larger ship, and which fled when it sank. 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.

Tuesday’s 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,” Choong said. “The criminal activities are flourishing because the risks are low and the rewards are extremely high.”

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 heading 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.

Of the 16 crew members, Wicharn said 15 are Thai and one is Cambodian.

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