The oil spill in the Gulf of Mexico is the worst in U.S. history, President Barack Obama's energy and climate czar said Tuesday.

Carol Browner, head of the White House Office of Energy and Climate Change Policy, said the administration is doing all it can to work with BP and Gulf Coast states to address the spill.

"I don't think there's any doubt, unfortunately," Browner said on ABC's "Good Morning America" when asked if the spill was the worst in history. 

No official estimate of the extent of the spill has been released at this point, partly because BP has only released official estimates and has not allowed independent measures. The spill's most directly comparable historical event was the 1989 Exxon Valdez spill in Alaska, which released some 10.8 million gallons into U.S. waters.

Browner said the administration was working with BP, which leases the wells that are leaking, to ensure the cleanup efforts are done in the most environmentally sensitive way. Concerns have been raised about the dispersant being used to break up the oil.

"EPA has looked at and will continue to look at all of the dispersants. As it turns out, there are not as many being manufactured as people thought in the quantities needed," she said. "What EPA did yesterday was direct BP to use less of this dispersant while they continue to study what other alternatives may be available.”

Browner said that BP has been entirely responsive to the government's orders on dispersants. 

Cross-posted to the E2 Wire.