The figures, which were announced on the State Oceanic Administration website earlier this week but only reported on Friday, were almost five times the size of the 840-square-kilometre area previously reported.
The administration says that area remains worst affected by the spill, but that another 3,400 square kilometres have also been contaminated to a lesser degree by the oil.
The spill was kept secret by the authorities for several weeks before being made public this month, sparking suspicions of an official cover-up, and the disaster has triggered a furious public response in China.
State media said the government was considering seeking compensation from Conoco Phillips over the spill.
"We have made an initial plan to claim compensation from Conoco Phillips China," the business daily 21st Century Business Herald quoted an unnamed official from the State Oceanic Administration as saying.
"But whether and how it will be implemented still depends on the status of plugging the leak."
CNOOC said last week the spill was "basically under control" while Conoco Phillips told reporters the leaks had been plugged.
But on Wednesday the oceanic administration said oil was still leaking into the ocean and ordered Conoco Phillips to stop operations at several rigs in the polluted area until the source of the spill was fully plugged.
"There has been oil seeping continuously into the sea for days from platforms B and C in the Penglai 19-3 oilfield and there is still a slick in the surrounding marine areas," it said in a statement.
"Another spill could happen at any time, which has posed a huge threat to the oceanic ecological 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.
Conoco Phillips said Thursday the spill was the equivalent of 1,500 barrels of oil.