China confirmed on Tuesday that China Oilfield Services Limited (COSL) had completed the first phase of oil drilling and exploration off Zhongjian Island of the country's Xisha Islands and has moved to another site for its second phase work.

Chinese Foreign Ministry spokesman Qin Gang told a press briefing the operation, carried out by the HYSY981 drilling platform managed by COSL, started on May 2 and is expected to be completed by mid-August.

"Chinese companies have carried out oil drilling operations offshore the Xisha Islands for 10 years," Qin said.

The current operation was a normal offshore drilling task in undisputed waters near the Chinese coast, which was justified and legitimate, he added.

According to a statement of the company, the waters for the first phase were 17 nautical miles from Zhongjian Island, completely within China's territorial waters, while they are as far as 130-150 nautical miles (241-278 km) from Vietnam's coastline.

COSL is a subsidiary of the China National Offshore Oil Corporation, the country's largest offshore oil and gas producer.