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 the Chinese unit of US oil giant ConocoPhillips, 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 Chinese 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 issued a statement Wednesday 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 840 square kilometres (336 square miles) -- larger than Bahrain. A Beijing-based spokeswoman for ConocoPhillips did not immediately respond to AFP requests for comment. Greenpeace on Wednesday slammed 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 news conference Tuesday that the slick was discovered on June 4, the state-run China News Agency 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.