China Oilfield Services Ltd., a unit of state-owned China National Offshore Oil Corp., said in a statement that its deep-water HYSY 981 drilling rig had completed exploration and drilling operations off Triton Island, or Zhongjian Island in Chinese. The island is part of the Paracel Islands chain, claimed by China and Vietnam.

The rig's operator, known as COSL, couldn't be reached for comment.

China's Foreign Ministry reiterated Wednesday that Chinese companies were within their rights to undertake work around the Paracel Islands. It said it opposed interference by Vietnam and had taken "necessary measures to safeguard the safety of operations" by COSL.

Vietnam protested Chinese drilling in the area after it began in early May and has sent coast guard vessels to confront the rig. Dozens of Chinese and Vietnamese maritime vessels had massed near the disputed rig, with each side claiming its ships had been violently rammed by the other's.

China's decision to deploy a rig to the contested waters drew criticism from Washington, which called it provocative. The episode touched off deadly anti-Chinese riots in Vietnam in May.

A Vietnam Coast Guard official confirmed that the oil rig was being moved.

"It started moving at 2100 [Tuesday] night and is moving toward China's Hainan Island," Rear Admiral Ngo Ngoc Thu, Vice Commander of Vietnam Coast Guard said.

Mr. Thu said that all the Chinese vessels in the area are moving with the rig. He said Vietnam was keeping its vessels at the site and is closely watching the oil rig's movement.

China National Petroleum Corp., the country's largest oil company by production, controls the disputed exploration area.

COSL said the rig would move to near Hainan island to continue exploration there and said it is planning to evaluate data collected during drilling at the contested site to consider its next phase of work.