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It says speedboats are being launched from ships that prowl the routes of the Indian Ocean, searching for targets.

Last week, a luxury cruise liner off Somalia's coast was attacked by pirates with rocket-propelled grenades.

The IMB says pirates are still holding seven ships and their crews, seized in the world's most dangerous waters.

In the past few days, at least four other vessels are reported to have been attacked.

'Out of control'

Captain Pottengal Mukundan, director of the IMB, says pirate attacks are being launched from at least two "mother ships".

Capt Mukundan says speedboats carry out the attempted hijacks before returning to the larger vessels floating at sea.

Crew turns to sonic weapon

This means even ships sailing far off the coast are vulnerable to attack.

He says the situation off the coast of Somalia appears to be completely out of control.

The IMB has recorded more than 30 hijack attempts in the region since March.

These latest attacks follow a thwarted attempt by pirates in small boats to commandeer the luxury liner, the Seabourn Spirit, which was steaming some 100 miles (160km) off the Somali coast last week.

But the liner's crew took evasive action, repelling the attackers without returning fire.

To scare off the pirates, they deployed a military-grade sonic weapon, capable of causing permanent damage to hearing from a distance of more than 300 metres (984ft).

Somalia has been without any effective government for 14 years.

Despite attempts to establish a new administration earlier this year, the country remains divided between rival warlords.