Work to clean-up an oil spill in East China's Shandong province has almost finished, according to a news conference on the incident by the State Oceanic Administration Tuesday.

The spill at Penglai 19-3 oilfield in Bohai Bay is under control now, said Li Xiaoming, an official with the bureau, who also admitted the leak had caused a "certain" degree of environment pollution to the surrounding area.

The leak was discovered early last month, according to a report from US energy company ConocoPhillips China, who is now operating the field under a joint development agreement with China National Offshore Oil Corp (CNOOC).

Approximately 840 square kilometers of Grade I seawater has been contaminated into the Grade IV, the worst quality type measured in the country, with the average oil concentration 40 times greater than the historical data.

The bureau did not specify the amount of the leaked oil, adding it needs further assessment.

It pledges to deal with the case seriously and will bring an accusation against the ConocoPhillips China who is responsible for the incident.