The Penglai 19-3 oilfield leaked up to 0.6 tons of oil into the sea on June 3 during a clean-up operation, according to a statement to China National Offshore Oil Corp Ltd released on Monday evening.

CNOOC Ltd said it received a report from oilfield operator ConocoPhillips China Inc that stated: "Oil leaked into the sea when the safety device of a hose coupling automatically parted because of an abnormality during the oil transfer under a fluid discharge and depressurization operation." The defect was quickly repaired.

CNOOC said it will keep a close watch on the incident and will actively assist ConocoPhillips with any related work. CNOOC holds a 51 percent stake in the field. The China unit of the Houston-headquartered oil major holds the remaining 49 percent interest.

The leak came just a year after an oil spill a year ago in China's northeast Bohai Bay. The country's biggest offshore oilfield leaked about 700 barrels of crude oil into the water on two occasions last June.

The State Oceanic Administration, China's top ocean watchdog, ordered the operator to suspend operations at the oilfield in September, citing negligence.

ConocoPhillips has submitted a revised overall development plan for the oilfield and an environmental impact assessment report to the top authorities as required. It has yet to receive approval to restart work.

The two companies announced they would pay more than 3 billion yuan ($474.50 million) in compensation as a result of the spill and toward marine environmental protection measures.