HAIKOU - Six sailors remain missing after their cargo ship from the Republic of Korea (ROK) sank off Xisha Islands in the South China Sea on Monday, Chinese rescuers said Tuesday.

The accident happened at about 6:40 pm Monday near Yongxing Island when the Bright Ruby sank with 21 crew members aboard, including nine ROK nationals and 12 Myanmar nationals, a spokesman with the Hainan Provincial Maritime Search and Rescue Center said.

Fifteen sailors have been rescued so far, while six others -- three Koreans and three Burmese -- were still missing, the spokesman said.

Currently, eight rescue boats, including two dispatched by Chinese authorities, are searching the sea area for the missing sailors, he said.

The cargo ship, which was carrying 16,000 tons of rolled steel, was en route to the city of Rizhao in east China's Shandong province from Singapore when the accident happened, he added.