It was not clear whether anyone in the crew, all from Myanmar, had been injured in the attack early Friday or whether any ransom had been demanded.
Svenn Pedersen, CEO and managing director of Norway's Th. Broevigtank Shipowners, said the UBT Ocean chemical tanker was on its way from the United Arab Emirates to Tanzania when it was hijacked about 300 miles (500 kilometers) from its destination. "The captain made contact saying pirates were on board and then the contact was cut off. Nothing has been heard from the ship since then," Pedersen said.
He said the 9,000-ton vessel is registered in the Marshall Islands and was carrying fuel oil.
Edward Ion, a spokesman for crisis management company Ship Management Associates in Singapore, said attempts have been made to re-establish contact with the vessel, but it has so far been unsuccessful.
"We're making every effort," he said, adding the company is cooperating with local authorities and monitoring agencies and anti-piracy forces in the region. "Obviously we're very concerned about this."
The hijacking is one of the southernmost attacks the pirates have ever launched, and serves as yet another indicator that increased naval patrols in the Gulf of Aden are pushing the pirates' range further south and east into the Indian Ocean.
The ship was not registered with maritime authorities, as recommended by industry guidelines. Ion said it is now heading north toward Somalia.