U.S. oil firm ConocoPhillips will gradually restart China's largest offshore oilfield after getting approval for a development plan and its environmental impact assessment, China's State Oceanic Administration said in a statement.

The field, Penglai 19-3, located in the northern Bohai Bay, was ordered shut in September 2011 after an oil spill. In January, approval was given for the development plan from the state energy authority, one of the steps necessary for a restart.

ConocoPhillips owns a 49 percent stake in the 168,000 barrel-per-day oilfield and acts as operator, while China's offshore specialist CNOOC Ltd has a 51 percent stake.

In the statement late on Saturday, the Administration said ConocoPhillips must strictly implement the requirements of the environmental assessment, while CNOOC should assist its partner in meeting the requirements.

The Administration will supervise the resumption of production, it said.

ConocoPhillips and CNOOC have previously agreed to pay 1.683 billion yuan ($267 million) in compensation for the oil spill.