State-controlled CNOOC discovered the leak on Friday and sent divers to survey and repair the damaged sub-sea pipeline at the Jinzhou 9-3 West oilfield, the company said in a statement.

"The company has been effectively cleaning up the oil sheen on the sea surface," CNOOC said, adding initial estimates indicated 0.38 cubic metres (13.42 cubic feet) of oil had leaked into the water.

CNOOC pledged in August to improve its "management system" to prevent further accidents after more than 3,000 barrels of oil and oil-based mud -- a substance used as a lubricant in drilling -- spilled into Bohai Bay.

CNOOC was accused of covering up that slick, which happened near platforms jointly owned by the Chinese company and US titan ConocoPhillips and polluted an estimated 5,500 square kilometres (2,200 square miles), according to Xinhua.

The State Oceanic Administration -- which supervises and manages China's seas -- has said it plans to sue ConocoPhillips over the spill, which was first detected in early June but only made public nearly a month later.