BEIJING - China's maritime authority started a site inspection on Thursday to check the claim of ConocoPhillips China (COPC) that it has sealed off an oil spill that polluted north China's Bohai Bay.

COPC, a subsidiary of US oil giant ConocoPhillips, said on Wednesday that it had sealed off the leaks before the August 31 deadline set by the State Oceanic Administration (SOA).

COPC said it had submitted a report to the SOA, showing that the company has met the SOA's requirements to seal off the sources of the spill and eliminate the risk for further leaks.

SOA confirmed with Xinhua late Wednesday that it has received the report but said the administration still needs to conduct site inspection and expert evaluation to verify the company's conclusions.

Hundreds of scallops are dumped in Luanjiakou port, in Penglai, Shandong province, on Aug 25. Sea farmers say their livelihood is under threat following an oil spill in Bohai Bay. Guo Xulei / Xinhua

The oil spill at the Penglai 19-3 oil field, China's largest offshore oilfield, was first spotted in June. It is "the most serious marine ecological incident in China," according to SOA.

A total of 5,500 square km of the bay's surface has been contaminated, with 870 square km seriously polluted, meaning that it is unfit for swimming and aquatic farming, according to SOA official Wang Fei.

Although the company has worked to clean up the spills, pollutants have still been found in the bay, even after cleanup efforts were reported to be complete.

The spills have spread to beaches in Hebei and Liaoning provinces and have been blamed for losses in the local tourism and aquatic farming industries. Aquatic farmers in Hebei planned to sue COPC and claim a total of 330 million yuan (51.56 million US dollars) in compensation, according to media reports.

SOA was also collecting evidence and gauging the ecological impact of the spills in preparation for possible legal action against COPC, SOA head Liu Cigui said last week.

COPC said it is conducting a thorough evaluation to determine the impacts of the incident on the environment but refused to give any comments on the losses it caused or possible compensations.

COPC said the report it submitted to SOA Wednesday includes the methods the company used to seal the leaks and third-party confirmation, but it did not give details.

The company "sincerely regrets these incidents in Bohai Bay and accepts its responsibility," COPC said in a statement posted on its website.

COPC estimates that 115 cubic meters of oil (700 barrels) and 400 cubic meters (about 2,500 barrels) of mineral oil-based drilling mud have leaked, it said.

The Penglai 19-3 oil field, which is jointly owned by COPC and China National Offshore Oil Corporation (CNOOC), China's largest offshore oil and gas producer.

COPC is the operator of the Penglai 19-3 oil field and owns a 49 percent stake while CNOOC has a 51 percent stake.