Pirates are again attacking ships in the busy Malacca Straits after a brief lull following the deadly tsunami which devastated Indonesia's Aceh province on Sumatra island last December, an international watchdog said today.

Three vessels were attacked in the key sealane in a two-week period, with pirates opening fire with automatic weapons to force the vessels to stop and allow them to board, the Piracy Reporting Centre of the London-based International Maritime Bureau said.

"The attacks, all thought to be made by the same gang, resulted in the kidnap of two or more seafarers from each vessel and were followed by a demand for ransom a few days later.

"The subsequent release of the hostages suggests that ransoms were paid," the report said.

In one of the incidents on March 14, pirates attacked a Japanese tugboat and kidnapped two Japanese and a Filipino, prompting Japan to request a military response from Malaysia and Indonesia.

Eighty percent of Japan's energy passes through the Malacca Straits, which is bordered by the Malaysian peninsula and Indonesia's Sumatra island.

Immediate action needed

Describing the attacks as reprehensible, IMB director Pottengal Mukundan said immediate action must be taken to capture and punish those responsible.

He noted that now that commerce had returned to normal in the tsunami-hit region, so had maritime crime.

The large international naval presence in the north that was providing aid in the devastated area had left and Mukundan warned that "unless something is done quickly, these kinds of attacks might escalate in frequency and violence".

Worldwide, pirates attacked 56 ships in the first three months of the year, with slow moving bulk carriers, tankers and cargo ships the main targets. Most of the attacks by armed gunmen took place in Indonesian waters.

Last year, 30 mariners were killed worldwide in pirate attacks, including four in the Malacca Strait, the IMB said.