Chinese media and green groups on Wednesday slammed the state-run China National Offshore Oil Corporation and the marine watchdog for keeping an oil spill hidden from the public for nearly a month.

CNOOC, in partnership with ConocoPhillips China, a subsidiary of the US oil giant, operates an oil field in Bohai Bay, off China's eastern coast, where the massive slick was detected on June 4 but only made public on Friday.

A strongly worded editorial in the Global Times newspaper accused the State Oceanic Administration (SOA) of protecting the oil giant, while the China Daily said CNOOC had an "obligation to share information".

"We cannot help but wonder: Is the SOA a serious watchdog that exists to prevent bigger incidents from happening, or a loving parent who is over-protective of his own child?" the Global Times editorial said.

Chinese-language newspapers also accused CNOOC of covering up the incident.

Such strident criticism of a large state-owned company is unusual in the Chinese media and could be a sign that Beijing is trying to boost its green credentials by showing the public it is getting tough on environmental abuses.

CNOOC vice president Chen Bi told a news conference the company was sorry for the negative impact of the spill but denied it had tried to cover up the incident.

"Ever since the founding of CNOOC we have never covered up any major oil spill," Chen told reporters.

CNOOC earlier issued a statement saying the spill was "basically under control" and that ConocoPhillips was "responsible for daily operations" of the oil field.

The SOA said Tuesday at the first government press briefing on the incident that it was probing the US firm's role in the slick, which has polluted an area measuring more than 800 square kilometres (300 square miles).

ConocoPhillips China president Georg Storaker told the news conference Wednesday that the company had notified authorities of the slick on June 4 and its priority since then had been "clean-up work".

"Both the authorities and ourselves are trying to get all the facts on the table -- this is a joint effort," Storaker added.

In an earlier statement, ConocoPhillips said "there is no oil sheen in the Bohai Bay operating area, the source of the sheen has been contained and clean-up work is close to completion".

Greenpeace lashed out at CNOOC, saying it had not learned lessons from last year's massive oil spill near the northeastern port city of Dalian, which the group said may have been 60 times bigger than reported.

Greenpeace said 60,000-90,000 tonnes of crude may have poured into the Yellow Sea in last year's incident after two pipelines exploded at an oil storage depot owned by another state oil giant, China National Petroleum Corp.

"The lessons we learned from that were companies need to release information and disclose whatever assessment they have in a very comprehensive and transparent way as soon as possible," Greenpeace activist Li Yan told AFP.

"But it seems the company has not learned from the Dalian event."

Li Xiaoming, head of the oceanic administration's Marine Environmental Protection Department, told the government news conference on Tuesday that the slick was discovered on June 4, the state-run China News Service reported.

He said the quality of water in the spill area was now at the worst level on the administration's four-grade pollution scale.