China has ordered US oil company ConocoPhillips to halt drilling at two leaking oil platforms in the Bohai Sea, with a warning that further spills posed "a huge threat to the oceanic ecological environment".

The injunction comes amid rising public anger over a slick that has spread at least 840 sq km, despite corporate reassurances that the situation was under control and government delays in releasing information.

It took a month for news to emerge about an initial seabed leak on 4 June in the Penglai 19-3 field, which is operated by ConocoPhillips in partnership with the China National Offshore Oil Corporation. Since then, there have been reports of two more leaks, most recently on Tuesday. Despite the companies' insistence last week that the spills were "basically under control", the State Oceanic Administration said on Wednesday that progress was slow and the situation remained worrying.

"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 administration said in a statement. "Another spill could happen at any time, which [poses] a huge threat to the oceanic ecological environment."

It said operations at the rigs must stop until the leaks are plugged and the risks eliminated. At an emergency meeting in Tianjin the previous day, Liu Cigui, the head of the SOA, called for tighter inspections of offshore wells, and checks on potential impacts from the pollution such as algae blooms and red tides.

In addition to the main slick, the government says an area of 3,400 sq km of the sea has suffered a sharp fall in water quality. The government has yet to publicly estimate the scale of the leak.