China said Tuesday it was investigating US oil giant ConocoPhillips' role in a spill off its eastern coast that authorities kept hidden from the public for nearly a month.

ConocoPhillips's China unit in a partnership with the state-owned China National Offshore Oil Corporation (CNOOC) operates the oil field in Bohai Bay, where the spill was detected on June 4 but only made public on Friday.

"The legal and supervisory maritime departments have opened a case on this oil spill and placed ConocoPhillips under investigation," the State Oceanic Administration said in a statement, noting that evidence was being collected.

An area measuring 840 square kilometres (336 square miles) had been badly polluted due to the spill, the statement said.

On Tuesday at the government's first news conference about the spill, Li Xiaoming, head of the ocean administration's Marine Environmental Protection Division, said the slick was discovered on June 4, the state-run China News Agency reported.

Li said oil had seriously contaminated ocean waters, with the quality of water in the spill area now at the worst level on the administration's four-grade pollution scale.

ConocoPhillips, which operates the Penglai 19-3 oil field where the leak originated, had been ordered to halt operations on June 13, Li said.

The spill has been controlled, but "small leaking points" continued to discharge oil into the ocean, Li said. The total amount of oil spilled is still being calculated, he added.

Dead seaweed and rotting fish could be seen in waters around Nanhuangcheng Island in Shandong province near the site of an oil spill, the China Daily said.

"The environmental impact caused by the oil leak is long-term," the newspaper quoted a local fisheries association official as saying.

Nanhuangcheng Island is about 75 kilometres (45 miles) from the offshore oil field in Bohai Bay where the leak happened.

CNOOC tried to stem anger over its failure to warn the public about the spill, saying government authorities were aware of the incident from the start.

"We reported the spills to authorities soon after they took place and treatment of the spills is under supervision," CNOOC spokesman Jiang Yongzhi was quoted as telling the Global Times.

The spill was first reported by a member of the public on the popular Chinese micro-blogging site Sina Weibo on June 21.