ConocoPhillips said it was still estimating the amount of oil that has escaped from the leak, which first came to light a month ago, and had uncovered evidence it may have been larger than it thought.
"During our cleanup operations in Bohai Bay, ConocoPhillips discovered additional oil-based drilling mud on the sea floor," the company said in a statement published on its website Wednesday.
"Although the amount of the discovery is unknown, we anticipate that it will push the total amount of oil and oil-based drilling mud released past the previous estimate of 1,500 barrels."
ConocoPhillips said it had reported the discovery to the authorities, and divers were continuing to work on cleaning up the sea bed.
Earlier this week China's State Oceanic Administration (SOA) accused the company of dragging its feet over the clean-up and threatened to fine it 200,000 yuan ($31,000).
The spill, which has polluted a sea area measuring 1,200 square kilometres, (460 square miles) according to the latest SOA figures, was kept secret by authorities for several weeks before being made public last month.
It had previously affected a much larger area of more than 4,000 square kilometres of ocean, according to figures provided by the government agency, which gave no explanation for the apparent contradiction.
The SOA has ordered ConocoPhillips to finish cleaning the contaminated ocean floor by Sunday -- a deadline the company said it hoped to meet.
"Hundreds of professional clean up workers are on the scene and we hope to meet the Sunday deadline," company spokeswoman Donna Xue rold AFP Thursday.
"Obviously, the new discovery puts extra pressure on the efforts."
ConocoPhillips and its local partner, the China National Offshore Oil Company, have apologised for the spill, but environmental campaigners have said they are not being transparent over the damage caused.
"It's hard to determine the fingerprint of the oil because this information is only accessible by the government," said Ma Jun, director of the Institute of Public and Environmental Affairs.
"The overall damage is still being assessed, but ConocoPhillips has done little to inspire trust."
Business owners have also expressed their anger about the spill.
A local fisheries association said this week that about 70 percent of the scallops farmed in the area were killed by the oil, causing losses of more than 200 million yuan.