BEIJING - A new oil leak was reported in northern China's Bohai Bay, the third since June.

The leak occurred at the Suizhong 36-1 oilfield at 1:30 am on Tuesday due to a malfunction at the central control system, China National Offshore Oil Corp (CNOOC) Ltd, the field's operator, said in a statement.

Technicians managed to seal the leak, which covered one square kilometer. Mats and chemicals were used to disperse the sheen which was likely to be cleaned up by Tuesday evening, the company said.

Production at the oilfield was suspended and emergency response procedures were activated immediately after the incident, CNOOC said.

The cause of the leak, a control system malfunction, had been repaired, it said.

Suizhong 36-1 is the country's biggest independent oilfield, annually producing about 5 million tons.

CNOOC's Tianjin branch is responsible for daily operations.

The State Oceanic Administration (SOA) said in a statement on Tuesday that it sent teams to monitor the situation after being alerted by CNOOC and dispatched a helicopter and used satellite remote sensing to monitor the spill.

The incident occurred on the heels of two other spills, in June, at the Penglai 19-3 oilfield in Bohai Bay, which polluted more than 840 sq km of coastal waters.

A cleanup operation is under way, according to ConocoPhillips China, the field's operator.

The US energy company holds a 49 percent stake in the project, while CNOOC holds the rest.

The two companies and the SOA faced widespread public criticism for covering up the Penglai spill for almost a month.

Both CNOOC and the SOA responded quickly to the latest incident on Tuesday.

The SOA said it will improve monitoring of offshore oil and gas activities and release information "accurately and promptly".

CNOOC said it will "continue to closely monitor production operations at all its oilfields" and reinforce procedures following a notice issued by the SOA, which requires offshore oil producers to identify spill risks and review emergency plans.

Regarding the Penglai 19-3 oil leak, the SOA said that the spill affected water quality across an area of 3,400 sq km. But it added that the polluted area has "dramatically decreased" during the past week.

"The leak at Platform B may continue to have minor residual seepage that we believe should diminish over time," ConocoPhillips China said on Tuesday.

"The latest incident at Suizhong oilfield is not as severe as the earlier ones in terms of volume," said Chen Jianmin, a professor at the School of Petroleum Engineering at China University of Petroleum. The company should enhance safety management and inspection procedures, he said.

"CNOOC is under comprehensive security checks at the moment after the incidents," said a company official, who declined to be named.

In another incident, a fire broke out at CNOOC's refinery in Guangdong province, located 40 km from the Daya Bay nuclear plant, on Monday. No casualties were reported.

CNOOC is the country's biggest offshore oil producer by capacity.