"The spill and the spreading is getting so much faster and expanding much quicker than they estimated," he told the Associated Press news agency.
The coast guard has said that an estimated 750,000 litres of oil is spilling from the site of the sunken Deepwater Horizon rig each day, threatening wildlife along the southern US coast.
But Admiral Thad Allen, a commandant of the coast guard, said it was difficult to estimate exactly how much crude was pouring from the broken riser.
"Any exact estimation of what's flowing out of those pipes down there is probably impossible at this time due to the depth of the water and our ability to try and assess that from remotely operated vehicles and video," he said.
As the scale of the disaster continues to unfold, pressure is mounting on the UK-based oil giant BP, the operator of the Deepwater Horizon rig, to step up its response.
The company has said it takes full responsibility and has deployed some 2,500 staff and is spending several million dollars a day trying to contain the spill.
Barack Obama, the US president, has already made clear that the oil firm will be expected to pay the cost of the clean-up.
On Sunday Obama is to head to the Gulf Coast region for a first-hand update on the leak and efforts to protect the environment of the area.
The announcement of his planned visit came as officials in the state of Mississippi joined Alabama, Florida and Louisiana in declaring a state of emergency.
|Video: What caused the slick
Al Jazeera looks at what engineers are facing in trying to stem the flow of oil
The US coast guard said on Saturday that the slick is likely to hit the Gulf Coast shoreline "at some point".
"There's enough oil out there, it's logical to assume it will impact the shoreline," Allen, the commandant of the coast guard, said.
"The question is where and when."
The sheen has already reached shoreline habitats on the southern US coast.
Recovery workers have been deploying miles of inflatable booms hoping to contain the oil, but strong winds and heavy seas have caused several of the barriers to break loose, washing them onto the shore.
Chemical dispersant is being used underwater in an attempt to tackle the oil as it leaves the broken riser, while remote-controlled subs were trying to activate a "blow out preventer" – a massive undersea valve that should have blocked oil from flowing into the sea when the rig sank.
With efforts to plug the leak so far unsuccessful and worsening weather in the coming days likely to further hamper attempts to plug the leak, fears are growing of an ecological disaster that could rival the 1989 Exxon Valdez tanker spill in Alaska.
That disaster spilled an estimated 40.9 million litres of crude oil into the previously environmentally pristine waters of Prince William Sound, devastating local wildlife.
|Fears are growing that the spill could devastate local fishing communities [AFP]
Already along the US Gulf coast the first birds coated in oil have been received by animal rescue centres, and wildlife groups are braced to receive many more casualties.
Federal and state officials have blamed the London-based energy giant BP for the spill, and they have questioned the company's ability to deal with the leak and the environmental damage it will cause.
"I do have concerns that BP's resources are not adequate," Bobby Jindal, the governor of Louisiana, said.
"I urge them to seek more help from the federal government and others."
Al Jazeera's Sebastian Walker, reporting from Venice, Louisiana, said BP's failure to control the spill has also raised questions about the oil company's plan.
"There doesn't seem to be a plan in place for such type of spillage. BP is certainly taking a lot of flak," he said.
"But it's also the government that people are asking questions of. They're saying 'Why wasn't this flagged up earlier?' The government only declared this an event of national significance on Thursday but this spillage started to happen on April 20.
"There is a lot of blame being thrown around and questions of whether this response has been quick enough."
BP under fire
The oil company has been accused of downplaying the risk of such a disaster in the Gulf of Mexico.
|Wildlife experts say millions of birds and other animals are under threat [Reuters]
In a 2009 report, BP suggested that such an accident leading to a giant crude oil spill and serious damage to beaches, fish and mammals was unlikely and virtually impossible.
BP has now taken full responsibility for the oil spill, promising to pay for the clean-up and compensate people who have been affected.
But the company said it still does not know how the accident happened.
"We actually don't know what caused this event and clearly the government has an investigation that they've initiated," Doug Suttles, a chief operating officer at BP, said.
"We've launched our own internal investigation as well, but since this event began, we've only had one focus, which is to stop the flow of oil and minimise the impact."