BEIJING -- China's State Oceanic Administration (SOA) said Friday that spills at an oilfield operated by ConocoPhillips in the Bohai Bay resulted from defects in the company's production and management.

ConocoPhillips had violated the original protocol of exploitation and failed to take necessary precautions when there were high risk of a spill, said a statement from the SOA, which has been investigating the cause of the accident since soon after it happened in early June.

Oil spills were found near platforms B and C of the Penglai 19-3 oilfield, polluting around 6,200 square km of water in the Bohai Sea, causing the country's worst ever incident of offshore maritime pollution.

The field, owned by the China National Offshore Oil Corporation (CNOOC), is being mined by ConocoPhillips China, the US oil giant's subsidiary, under a joint development agreement with the CNOOC.

Investigations showed that ConocoPhillips was supposed to apply an oil production system named separate zone water flooding technology, but failed to do so, which increased the sea bed's instability near platform B and caused a spill on the sea bed, the statement said.

Before the accident happened, the operator had detected pressure changes at the exit of a water flooding well on platform B, a sign for possible flaws, but failed to find out the cause and take necessary precautions, the statement said.

In addition, the company failed to follow the proper drilling protocol on platform C, leading to an oil spill near the platform, the statement said.

On Oct 24, Chen Bi, vice-president of the CNOOC, told Xinhua that all oil spill sources have been identified and blocked off at ConocoPhillips' oil platforms and no new leak source had been detected in the sea.

Clean-up work has been going on, he added.

According to the latest SOA monitoring report on Wednesday, slight oil spills were still detected in the sea area near platform C on Tuesday.