Authorities confirmed that an offshore oilfield in Bohai Bay had spilled oil but they have "effectively controlled" the leak.

A report from ConocoPhillips China (CPOC), the operator of the Penglai 19-3 oilfield, said an oil sheen on the sea surface near the oilfield was observed "in early or mid June", a spokesperson for China National Offshore Oil Corp (CNOOC) Ltd said in an e-mailed statement on Friday.

CPOC took immediate action to address it, including the deployment of equipment and trained personnel to contain and clean up the sheen, an e-mailed statement from the US-based company said.

The source of the spills has been contained and cleanup work was close to completion. The work was performed under the supervision and guidance of authorities, including the State Oceanic Administration.

No injuries or reports of an impact on wildlife, fishing or shipping activities have been issued.

Penglai 19-3, off the coast of North China, was discovered in 1999 and is the biggest offshore oil discovery in China. It is operated by ConocoPhillips China and China National Offshore Oil Corp owns a 51 percent equity interest.

The cause of the oil spills was still under investigation, according to the statement released by CNOOC, which did not mention the amount of oil leaked, the date of the leakage or the size of area affected.

But insiders said not a large amount of crude oil was leaked from two locations, one around the B Platform and the other in nearby waters around the other platforms, a Southern Weekend report said.

The insiders also thought huge amounts of water were injected into the layer, creating excessive pressure that ruptured it, leading the oil spills.

The State Oceanic Administration will hold a news conference to release its assessment of the oil spills on July 5 and details about them will be released, an official from the administration told China Daily on Friday.