Five times as much oil as previously thought could be leaking from the well beneath where a rig exploded and sank last week, the US Coast Guard says.
Rear Admiral Mary Landry said some 5,000 barrels (210,000 gallons) a day were thought to be gushing into the sea 50 miles (80km) off Louisiana's coast.
The slick is 45 miles by 105 miles and is heading towards the coast.
If the coastguard estimate is correct, within two months the spill could match the 11 million gallons spilt from the Exxon Valdez tanker off Alaska in 1989.
Oil giant BP operated the Deepwater Horizon rig. Its chief operating officer of exploration and production, Doug Suttles, welcomed the US military's offer of help.
He said the company was using remote operative vehicles (ROVs) to try to find out how much oil was leaking into the sea.
"This is very, very difficult to estimate," Mr Suttles told reporters.
"Down below the surface we actually can't meter this oil so we can just observe it... what our ROV pictures show to us on the sea floor hasn't changed since we first saw the leak... but what we can say based on what we're picking up on the surface it looks like it is more."
Mr Suttles estimated something between 1,000 and 5,000 barrels a day was leaking.
Meanwhile, a firefighting expert said the disaster may become the biggest oil spill ever.
Mike Miller, head of Canadian oil well firefighting company Safety Boss, told the BBC World Service: "Probably the only thing comparable to this is the Kuwait fires [following the Gulf War in 1991].
"The Exxon Valdez is going to pale in comparison to this as it goes on."
Scientists say only a quarter of local marine wildlife survived the Exxon Valdez disaster.