Eleven environmental organizations have sent an open letter to ConocoPhillips China and the China National Offshore Oil Corp (CNOOC) Ltd calling for a "full apology for the concealment of recent oil leaks and their failure to immediately disclose the specifics of the incidents."

The letter that was sent on Monday was jointly written by Green Beagle, Friends of Nature, the Center for Legal Assistance to Pollution Victims, Nanjing Green Stone, Green Han River, Red Phoenix Project, Green Watershed, Shanghai Healthy Consumers Purchasing League, Friends of Green in Tianjin, Green Anhui and the Institute of Public and Environmental Affairs.

The letter claims the CNOOC concealed oil spills that blighted Bohai Bay and says the organization violated the requirement of a listed company to disclose information and the basic principles of corporate social responsibility.

The letter also calls for a full apology for polluting the environment from ConocoPhillips China, the company that the groups say was directly responsible for causing the leak. The groups also want ConocoPhillips China to apologize for allegedly concealing the incident.

"We also call on the two companies to make it possible for NGOs and the public to visit the scene of the leak, so they can see whether the oil spills have been controlled and see whether there is any impact on the marine environment," Feng Yongfeng, founder of the Chinese environmental organization Green Beagle, told China Daily on Wednesday.

Ma Jun, director of the Institute of Public and Environmental Affairs, agreed that the area should be open to the public and said the environmental impacts caused by the oil leak are likely to be long-term and could potentially harm the local fishing industry.

ConocoPhillips China apologized for the oil leak during a press conference on Wednesday. CNOOC also expressed its regret at the same gathering.

However, neither company has offered an estimate of how much oil was leaked nor given an explanation of exactly what happened.

The press conference was the first official acknowledgement of the leaks by the two companies since oil pollution was detected on June 4.

Although ConocoPhillips reported a small oil leak from an unknown source near the Penglai 19-3 oilfield to the North China Sea Branch of the State Oceanic Administration on June 4, the public and media was not told and did not find out about it until it was exposed on Sina Weibo, a popular Chinese micro-blogging site, on June 21.

Speculation about the size of the spill was rife online and in the media - with guesses ranging from 200 square meters to "3 kilometers long and 20 to 30 meters wide" - before the press conference organized by the State Oceanic Administration on Tuesday. It was then revealed that 840 square kilometers had been seriously polluted as of Monday.

The Penglai 19-3 field is China's largest offshore oilfield and produces 160,000 barrels a day. ConocoPhillips holds a 49 percent stake in the project while CNOOC has 51 percent.

"Hiding information from the public will damage the corporate image of the two companies because, in the new media age, no secret can be hidden forever," said Xia Xueluan, a sociologist at Peking University.

He added that the oil leak was not an internal issue for the companies but a matter of public safety and should have been publicized immediately.