Pirates have attacked a gas tanker in the Malacca Strait, kidnapping its captain and chief engineer for ransom, a watchdog said.
The Malacca Strait is notorious for pirate attacks
The International Maritime Bureau (IMB) said about 35 pirates with rocket launchers stormed the Indonesian-owned MT Tri Samudra late on Saturday.
They diverted the Belawan-bound vessel to a different part of Sumatra, before disappearing with the hostages.
The shipowners are trying to negotiate the hostages' release, the IMB says.
The narrow Malacca Strait between Indonesia and Malaysia is one of the world's busiest shipping lanes and has long been a haunt of pirates.
Some 37 acts of piracy were recorded there last year but there has been a sharp fall in reported attacks after December's tsunami in the Indian Ocean, the Malaysia-based bureau says.
Some experts say the giant waves could have killed many pirates and destroyed their boats, while others believe the large presence of international troops in the region has deterred attacks.
The 1,289-ton MT Tri Samudra was carrying a cargo of methane gas from Samarinda in Borneo island to Sumatra's Belawan when it was stormed.
"The pirates attacked the ship... and ordered it to sail to Dumai," IMB's regional manager Noel Choong told the AFP news agency.
"During the journey to Dumai the captain and engineer were kidnapped and taken off the ship," Mr Choong said.
He added that the vessel's owners believed the hijackers could be rebels from the Free Aceh Movement (Gam), which has been fighting for independence for Sumatra's northern Aceh province.
"If this is a terrorist attack it will have severe consequences on the security of the ports in the region," Mr Choong said.
"It looks like they [pirates] are becoming very daring and they are moving away from the... coastal attacks towards the one sea and towards Malaysian waters," he said.