China has sent a new oil rig to explore the East China Sea, an area that includes waters disputed with Japan.
Cosco Shipyard, which built the rig, did not disclose the exact location of the Kaixuan-1, or Triumph-1, drilling platform.
It is not known if it is in an area close to the disputed waters.
Cosco Shipyard said the rig was handed over to China Oilfield Services on July 17. China Oilfield Services and sister company China National Offshore Oil did not respond to requests for comment.
China and Japan have been at loggerheads over exclusive economic zones in the sea. China said it discovered an undersea natural gas field known as Chunxiao within its exclusive economic zone in 1995.
Japan also said it might tap into fields that stretch into the disputed area.
Both sides agreed in 2008 to jointly develop the Chunxiao field but there has been no progress since then.
A Cosco Shipyard statement said the Kaixuan-1 had a smooth start to drilling operations despite challenges thrown up by typhoons.
The drilling went to a depth of 5,200 metres, according to the statement.
China and Japan have engaged in a bitter dispute in the East China Sea over sovereignty of the Diaoyu Islands, known as the Senkakus in Japan. The risk of military confrontation was heightened as both nations sent ships and aircraft to the area.
Beijing has locked horns with its neighbours over territorial disputes in recent years, including with the Philippines and Vietnam over contested areas in the South China Sea.
Deadly anti-Chinese protests erupted in Vietnam in May after Beijing deployed an oil rig near the Paracel Islands. The oil rig was removed one month ahead of schedule in July, with Beijing saying it was because its work was completed.