In a tense standoff, BP continued to spray a product called Corexit in the Gulf of Mexico on Monday to break up a vast oil spill despite a demand by federal regulators that it switch to something less toxic.

The Environmental Protection Agency had set a Sunday night deadline for BP to stop using two dispersants from the Corexit line of products. The oil company has defended its use of Corexit and taken issue with the methods the agency used to estimate its toxicity.

At a news conference Monday, the E.P.A. administrator, Lisa P. Jackson, said that she was “dissatisfied with BP’s response” and had ordered the oil giant to take “immediate steps to scale back the use of dispersants.”

Ms. Jackson called BP’s safety data on dispersants insufficient and said government scientists would conduct their own tests to decide which dispersant was best to use. She said the amount of chemicals applied to control the oil spilling from the Deepwater Horizon well — more than 700,000 gallons so far on the gulf’s surface and a mile underwater at the leaking well head — was “approaching a world record.”