A massive oil spill off China\'s northeast coast was far more extensive than first thought, the US oil giant behind it said in a statement Friday, pledging to clean it up by the end of August. ConocoPhillips said more than 2,100 barrels of oil and oil-based mud -- a substance used as a lubricant in undersea drilling -- had leaked from two platforms in the oilfield it operates with a Chinese partner in Bohai Bay. This amount is well above the 1,500 barrels the Houston-based company said on July 14 had leaked after the problem was first detected in early June. China\'s government-run State Oceanographic Administration has levelled strong criticism at the fifth largest US oil company for its \"inefficient and temporary\" response to the disaster. Environmental activists have also weighed in, but the China National Offshore Oil Corporation, ConocoPhillips\' partner in the Bohai drilling, has largely escaped criticism. Friday\'s statement, published on ConocoPhillips\' website, said that \"only trace amounts of oil have been found and recovered from shorelines\". \"ConocoPhillips China is making significant progress on cleaning up oil-based drilling mud from the seabed,\" Georg Storaker, president of the company\'s China unit, said in a statement. \"We have already cleaned up approximately 70 percent of the mineral oil based mud on the seabed and will have the additional volume cleaned up by the end of August.\" Earlier, the company said it had hoped to have the clean-up finished by August 7, but the operation was delayed by heavy seas caused by Typhoon Muifa and the discovery of additional oil on the seabed. Fishermen in the Shandong, Hebei and Liaoning provinces that border Bohai Bay, due east of Beijing, allege that oil from the leak has killed a large part of their harvest of such seafood as scallops. Eleven environmental groups signed an open letter to ConocoPhillips last week calling for speedier and more transparent clean up efforts.