ConocoPhillips China (COPC), a subsidiary company under the U.S. energy giant ConocoPhillips, "should take the blame" for the oil leak near the Penglai 19-3 oilfield in Bohai Bay in north China, the State Oceanic Administration (SOA) said Tuesday.

The incident, which took place last month, has polluted an area of 840 sq-kms, causing "a certain level" of damage to the oceanic environment, said the administration.

The field is under the operation of COPC under a joint development agreement with the China National Offshore Oil Corporation, the country's largest offshore oil producer.

According to the administration, the leak from platform-B resulted from increased pressure of strata when workers injected water into the strata.

The leak from platform-C was due to a surge in the well coupled with leaks on the side, said the SOA, without giving details.

As of Monday, a total of 70 cubic meters of water-oil hybrid in the area had been cleaned up, but "a small amount" of oil film can still be seen on the sea surface, according to the SOA.