Oil has been gushing from the well for the past five weeks BP has slowed the flow of oil and gas from a ruptured well into the Gulf of Mexico, a US official told local media.
The company's "top kill" effort has "stabilised the wellhead", Coast Guard commander Admiral Thad Allen said.
But he cautioned it was too early to declare success. This is the first step in BP's plan to seal the well for good.
Footage from BP's underwater camera
Adm Allen told US media the "top kill" procedure, which began on Wednesday, has pumped enough drilling fluid to block some of the oil and gas escaping from the well.
Adm Allen told National Public Radio that BP engineers had "been able to force mud down and not allow any hydrocarbons to come up."
It was the first positive official assessment of BP's latest attempt to plug the well, after previous efforts failed.
BP shares were up more than 5% in London trading following the announcement.
BP has not yet commented in detail on the situation, saying merely that its "subsea efforts [were] advancing on several fronts".