China said Wednesday it had ordered US oil giant ConocoPhillips to immediately stop operations at several rigs in an area off the nation's eastern coast polluted by a huge slick.

The 840-square-kilometre (336-square-mile) slick emanating from the oil field in Bohai Bay -- which ConocoPhillips operates with China's state-run oil giant CNOOC -- has caused huge anger amid allegations of a cover-up.

On Wednesday, the State Oceanic Administration (SOA) said operations would not be allowed to resume before the source of the spill was fully plugged and "risks eliminated", as fears over the long-term impact on the environment grow.

"There has been oil seeping continuously into the sea for days from platforms B and C in the Penglai 19-3 oil field and there is still a slick in the surrounding marine areas," the SOA said in a statement.

"Another spill could happen at any time, which has posed a huge threat to the oceanic ecological environment," it said, adding it had ordered the US firm to stop operations at those platforms.

CNOOC said last week the spill -- which was detected on June 4 but only made public at the beginning of July -- was "basically under control" while ConocoPhillips told reporters the leaks had been plugged.

The official China Daily newspaper said last week that dead seaweed and rotting fish could be seen in waters around Nanhuangcheng Island near the site of the slick.

It quoted a local fisheries association official as saying the oil leak would have a "long-term" impact on the environment.

CNOOC has been slammed by state media and green groups over the spill, and it emerged on Tuesday that the firm was cleaning up another slick after a breakdown at a rig off the northeast coast.

The state-run giant said immediately the leak was "minor".

In a separate incident, a CNOOC refinery in the southern province of Guangdong caught fire Monday but there were no casualties, the company said, adding the cause of the blaze was still under investigation.

The refinery is located about 40 kilometres (25 miles) from the Daya Bay nuclear power plant, according to the official Xinhua news agency.