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.
Meanwhile, a panel of US scientists said the oil leak was much worse than previously estimated, making it the nation's worst spill.
US Geological Survey Director Marcia McNutt saidBP 'slows leaking Gulf oil well' government teams estimated the flow ranges from 12,000 barrels (504,000 gallons or 1.9 million litres) to 25,000 barrels per day.
Up to now, BP had estimated the leak at 5,000 barrels (210,000 gallons) per day, while cautioning that figure was unreliable.
If the new estimates are confirmed, it would mean the leak has far eclipsed the previous worst oil spill in US history - the 1989 Exxon Valdez disaster.
Later on Thursday, President Barack Obama is expected to extend a moratorium on deep-water offshore drilling for six months, the White House has said.
The move comes as his administration faces criticism of its handling of the Gulf of Mexico oil spill.
Hours before Mr Obama was due to speak, US officials announced the resignation of the head of the Minerals Management Service (MMS), the US agency which oversees drilling operations.
Elizabeth "Liz" Birnbaum, who had run the MMS since July 2009, has endured criticism in the weeks since the oil spill over alleged lax oversight of drilling and what President Obama has called an overly cozy relationship with industry.
Eleven workers were killed in the explosion on the Deepwater Horizon rig on 20 April. Millions of gallons of oil have poured into the sea since then.
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.
Earlier, he had indicated that all the leaking oil and gas had been stopped, telling National Public Radio that BP engineers had "been able to force mud down and not allow any hydrocarbons to come up."
An aide to Adm Allen later clarified his statement, saying the procedure had a way to go before it proved successful.
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".
The company did not know how long the operation would take, a BP official said.
The top kill attempt, which began on Wednesday, involves pumping heavy drilling fluids into the top of the well to try to halt the oil flow.
If it succeeds, cement would then be injected to seal the well.
The oil leak has already soiled more than 110km (70 miles) of Louisiana's coastline, threatening fragile marshlands and putting the Louisiana fishing industry at risk.
US officials warn the coming hurricane season could make things worse.
The National Oceanic and Atmospheric Administration (NOAA) forecasts 14 to 23 named storms, with 8 to 14 developing into hurricanes, making it one of the most active on record.