Chinese authorities have detained 12 Indonesians for allegedly hijacking an oil tanker, siphoning off its petroleum and attempting to smuggle its cargo into China, the ship's agents said Tuesday.

The Malaysian-registered Petro Ranger tanker was reported missing while sailing to Vietnam with a cargo worth $1.5 million and was discovered in the southern Chinese port of Haikou on Hainan Island 11 days later.The vessel left Singapore on April 16 bound for Ho Chi Minh City with about 11,000 tons of diesel and kerosene and was hijacked by gun-wielding pirates one day into the three-day voyage.

Chinese authorities in Hainan had said the original 21-member multinational crew was still aboard the Petro Ranger when it was detained - along with 12 Indonesians who are now in police custody, Capt. Johnny Liew of Singapore shipping agent Petroships said today.

"They are under suspicion of hijacking and smuggling," he said.

Petroships and International Maritime Bureau representatives have been joined by Malaysian and Australian consuls on the scene.

Most of the cargo was discharged to other barges, Liew said, but officials don't know how the transfer took place. They also don't know what countries the rogue ships came from.

The ship's communications system was damaged during the hijacking, and its name was painted over with the name "Wilby," said Liew.

Crew members were given medical checks on Monday, said Li Xueling, a spokesman for Hainan's Foreign Affairs Office, but no health problems were found.