China has told US oil giant ConocoPhillips to step up its efforts to seal leaks and clean up a spill off its north coast after finding oil was still leaking into the sea, state media reported Friday.

ConocoPhillips said last week it had halted production at its Penglai 19-3 oil field, China's biggest and the source of a June leak into Bohai Bay that has caused public anger and led to accusations of a cover-up.

But China's State Oceanic Administration (SOA) said it had carried out checks over the past week and found that oil was still leaking near a drilling platform in the field.

"(The platform) is continuing to show this phenomenon (of leaking) after a relatively long period of time. This shows the source has not been effectively sealed," the state Xinhua news agency quoted an unnamed SOA official as saying.

The SOA called on ConocoPhillips and its Chinese partner, CNOOC, to "continue to thoroughly seal the oil source and continue to clean up oil pollution on the sea," it said.

Conoco said the oil detected was from "mineral oil based mud" -- a lubricant used to facilitate drilling -- lying on the sea bed.

"Small quantities of mineral oil based mud from the June 17 incident remain on the sea floor pending clean up," a company statement said.

"These residual materials occasionally release small amounts of oil which rise to the surface and create small sheens on the surface. A small sheen was spotted yesterday and cleaned up."

The SOA, which supervises and manages China's seas, has said it plans to sue ConocoPhillips and seek compensation for the spill, which state media said polluted at least 5,500 square kilometres (2,124 square miles) of sea.

The spill was first detected in June and the Chinese government had given ConocoPhillips until the end of August to complete the clean-up and stop any remaining leaks.

China's cabinet last week ordered a thorough investigation into the spill and vowed to punish those found responsible.

ConocoPhillips, the third-largest oil firm in the US, has apologised and said it was setting up a fund to address the company's legal responsibilities under Chinese law and to benefit the environment of Bohai Bay.