The International Maritime Bureau (IMB) said the tanker was sailing from Singapore to Cambodia when it was boarded by 16 pirates about 26 nautical miles off Aur Island on the east coast of the peninsula late April 17.
The pirates, armed with swords and guns, damaged the ship's communication equipment, transferred part of the fuel cargo into a smaller unknown tanker and stole the ship and crews' belongings before fleeing, said Noel Choong, who heads the IMB's piracy reporting center in Kuala Lumpur.
A similar theft followed just five days later. The pirates stopped a Singapore tanker in the Malacca Strait off Malaysia's west coast and stole part of its diesel cargo worth US$2.5 million (81.25 million baht).
''It is the same modus operandi and could be the work of the same syndicate targeting the same cargo that left from the same port,'' Mr Choong said. ''It's a concern that these two incidents happened in a short time. We hope the authorities can nip it before it gets worse.''
He said the captain of the Thai ship sustained minor injuries. The ship continued its journey and reached Cambodia on April 20 but only reported the theft to the IMB on Wednesday, he said.
The IMB has warned ships to maintain a strict piracy watch.
Three Indonesian crew members of the Singapore tanker, including the captain and chief engineer, disappeared along with their belongings during the pirate attack. Authorities are investigating if they were abducted or were involved in the sea robbery.
Piracy, mainly low-level thefts, had been declining following maritime patrols by the three countries, but attacks appeared to have picked up last year in Malaysian and Indonesian waters.