Oil leaks at China's largest offshore oilfield in Bohai Bay, operated by US energy giant ConocoPhillips Co, may be continuing as the company has failed to take effective remedial measures, according to the State Oceanic Administration (SOA), the country's ocean watchdog.

While an on-site investigation of the Penglai 19-3 oilfield found no oil slicks near the company's B and C platforms on Friday, agencies under the SOA found minor seepages were still occurring 20-30 times a minute, leaking about 1-liter of oil in 24 hours at Platform C, where an open-flame was burning on the platform.

Operating staff at ConocoPhillips claimed the flame was caused by the burning of natural gas to generate electricity for the platform and Lu Aimin, manager of Platform B, told the Xinhua News Agency the seepages at Platform C were caused by an effort to clean up oil on the seabed.

Lu also said the leak at Platform B is now under control as the company had deployed an oil-containment fence and the SOA is conducting hourly inspection tours to monitor the platform.

But Lin Fangzhong, deputy director of the northern seas branch of China Marine Surveillance, told Xinhua the seepages might be the result of inadequate cement plugs at known leak points or possibly undetected leaks.

"The reason for this is still unknown and ConocoPhillips should make further efforts to find out the facts as required by the SOA," Lin was quoted as saying.

Officials at the SOA confirmed ConocoPhillips had still not submitted its report on the cause and location of the leaks as ordered and that, so far, more than 4,240 square kilometers of coastal waters have been contaminated by the oil leaks.

Cui Wenlin, director of the North China Sea Environment Monitoring Center, told Xinhua that according to the results from four monitoring stations - Dalian, Qinhuangdao, Tianjin and Yantai - the oil spill has not yet contaminated the shores of Bohai Bay.

However, a huge tidal belt of red foam was observed at 2 pm on Friday, situated about 3 nautical miles off Platform B and 2 nautical miles off Platform C. Cui, who joined up with the investigation team to visit the scene, took a sample of the water for "further analysis and observation".

Eleven environmental organizations plan to file a joint lawsuit against ConocoPhillips and its partner, China National Offshore Oil Corp.