China's maritime authorities said Tuesday that two oil spills that occurred near the country's northeastern coast last month have had a negative effect on water quality in a "broad sea area" in addition to seriously polluting 840 square km of sea area.

An area of 3,400 square km in China's Bohai Bay has been affected by the spills, with the area's water quality rating slipping from level I to levels III and IV, the North China Sea Branch of the State Oceanic Administration (SOA) announced Tuesday in a press release.

The oil spills, which occurred in the bay's Penglai 19-3 oilfield, have also seriously polluted 840 km of sea area in the bay, sending water quality ratings in the area to their lowest level. Spilled oil has been detected in sediment samples taken from areas near the oilfield, according to the press release.

The oilfield is home to oil platforms operated by ConocoPhillips China (COPC), a subsidiary of U.S. energy giant ConocoPhillips, under a joint development agreement with the China National Offshore Oil Corporation (CNOOC), the country's largest offshore oil producer.

While the COPC said last week that the oil spills have been contained and that clean-up efforts have nearly been finished, a statement issued by the SOA on Monday said that investigators found a "small amount" of oil leaking from one of the oilfield's platforms.

The first oil spill was reported to the SOA North China Sea Branch by ConocoPhillips on June 4, with the second spill reported on June 17. The leaks were brought under control by June 19 and 21, respectively.

The first spill occurred when ConocoPhillips workers were drilling into the seabed near the oilfield. The increased pressure from their drilling operations caused oil to be released from the seabed before it could be contained. The second spill was caused by an oil well surge, according to the SOA. No further details of the second spill were made available.

The SOA said the COPC will be fined 200,000 yuan (30,770 U.S. dollars) for the oil spills.

However, Wang Bin, a senior official with the SOA oceanic environmental protection bureau, said maritime authorities will also claim environmental compensation from the COPC in accordance with relevant laws. He added that the amount of compensation will be "much more" than 200,000 yuan.

Yet another oil spill occurred around 1:30 a.m. Tuesday in the CNOOC's Suizhong 36-1 oilfield, which is also located in Bohai Bay. The spill was caused by a breakdown in the control system of the oilfield's central platform, according to a SOA press release that cited a report submitted by the CNOOC's Tianjin branch.

It is estimated that about 0.1 to 0.15 cubic meters of oil leaked out of the platform during the spill, polluting an area of 1 square km, the press release said.

The CNOOC is using oil-absorbent mats and oil dispersants to clean up the spill, while the SOA has dispatched a helicopter and is using satellite remote sensing to monitor the situation, the press release said.