An oil leak near the Penglai 19-3 oilfield in Bohai Bay in north China has been largely cleaned up, the country's largest offshore oil producer, China National Offshore Oil Corporation (CNOOC), said in a statement on Friday.

The leak was discovered early last month, the statement said, citing a report from U.S. energy company ConocoPhillips China, who is now operating the field under a joint development agreement with CNOOC.

The cause of the leak remains under investigation, the statement said, without mentioning the amount of oil leaked or the level of environmental damage.