Rear Admiral Mary Landry said 5,000 barrels (210,000 gallons) a day were now thought to be gushing into the sea 50 miles off Louisiana's coast. A third leak had also been discovered at the site, Admiral Landry said.
Earlier, a Coast Guard crew set fire to part of the oil slick, in an attempt to save environmentally fragile wetlands. The "controlled burn" of surface oil took place in an area about 30 miles (50km) east of the Mississippi river delta, officials said.
Weather forecasters have meanwhile warned that changing winds could drive the oil slick ashore by Friday night.
Admiral Landry said experts from the National Oceanic Atmospheric Administration (NOAA) had revised up their estimate for the leak based on aerial surveys, applying dispersants, studying the trajectory of the slick, local weather conditions, and other factors.
"This is not an exact science when we estimate the amount of oil. However, the NOAA is telling me now they'd prefer we use at least 5,000 barrels a day," she told reporters in New Orleans.
Admiral Landry also said she had been told of "a new location of an additional breach in the riser of the deep underwater well", about 5,000ft under the surface.
President Barack Obama had been briefed on the new developments, and the government had offered to have the defence department help contain the spill, she added.
Louisiana Governor Bobby Jindal has requested emergency assistance from the federal government.
"Our top priority is to protect our citizens and the environment. These resources are critical to mitigating the impact of the oil spill on our coast," he said in a statement.