BEIJING, July 16 -- A drilling operation by the HYSY 981 rig off the Xisha islands in the South China Sea has been completed, China's Foreign Ministry confirmed on Wednesday.

Quoting China National Petroleum Corp. (CNPC) and China Oilfield Services Limited, ministry spokesman Hong Lei said the Chinese companies began the drilling operation off Zhongjian Island, one of the Xisha Islands, on May 2. Work was completed on July 15.

The CNPC has been exploring the area since 2004.

According to the spokesman, the drilling rig is set to continue its work off south China's island province of Hainan after the Xisha operation.

The relocation of the operation is in line with the companies' own arrangement and is not due to any external reason, he said.

Hong said the companies will analyze and assess the geological data collected and decide on the next step.

The spokesman also noted the Xisha Islands are inherent territory of China and that the operation was conducted in indisputable inshore waters off Xisha, which completely falls within China's sovereignty, sovereign rights and jurisdiction.

"China strongly opposes Vietnam's irrational disruptions of Chinese companies' operations and has taken necessary security measures to ensure the operation," Hong added.