President George Bush has conceded the initial response to Hurricane Katrina was "not acceptable" but has said every effort is being made to save lives.
Thousands of people remain stranded without food or water in New Orleans, where extra troops have been dispatched to quell lawlessness.
Fires are raging in the city after huge blasts apparently at a chemical plant.
Arriving in the area, Mr Bush said New Orleans would emerge from its "darkest days" but much work was to be done.
"My attitude is, if it's not going exactly right, we're going to make it go exactly right. If there's problems, then we'll address the problems," Mr Bush said.
The president, who is expected to go on to visit Mississippi and Louisiana - but not New Orleans itself - was briefed by emergency officials on landing in Mobile in Alabama.
He said: "The immediate priority is to save lives and get food and medicine to people so we can stabilise the situation."
Thousands are feared to have perished in the hurricane and floods, or while waiting for help.
The Senate has approved $10.5bn (£5.7bn) emergency aid, which the House of Representatives is expected to back within the next 24 hours.
But the head of the New Orleans emergency operations described the relief effort as a national disgrace.
And Mayor Ray Nagin has angrily denounced the level of outside help the city has received. "People are dying here," he said.
A large cloud of acrid, black smoke is drifting over New Orleans following Friday's blast along the Mississippi riverfront.
The incident in the already crippled city came after Louisiana's governor said 300 "battle-tested" National Guardsmen were being sent to quell the unrest.
"They have M-16s and are locked and loaded. These troops know how to shoot and kill and I expect they will," Kathleen Blanco said.
Washington pledged a further 4,200 guardsmen in coming days, and said that 3,000 army soldiers may also be sent to the city where violence has disrupted relief efforts.
The deployment came as thousands were finally taken from the Louisiana Superdome, where up to 20,000 have been corralled amid heat and squalor since Katrina struck.
The BBC's Matt Frei, in New Orleans, says conditions in the city's convention centre, where up to 20,000 more are stranded, are the most wretched he has seen anywhere, including crises in the Third World.
"You've got an entire nursing home evacuated five days ago - people in wheelchairs sitting there and slowly dying," he says.
The situation has been made worse by a lack of trust between the mainly poor, African-American population left behind in New Orleans and the predominately white police force, our correspondent adds.
Up to 60,000 people could still be stranded in the city, the US coastguard says.
Looting has swept the city as people made homeless by the flooding have grown increasingly desperate.
There have also been outbreaks of shootings and carjackings and reports of rapes.
The federal emergency agency was trying to work "under conditions of urban warfare", director Michael Brown said.
The muddy floodwaters are now toxic with fuel, battery acid, rubbish and raw sewage.
'Blame game'
Residents have expressed growing anger and frustration with the disorder on the streets and with the slow speed of relief efforts.
Governor Blanco told ABC she had "no idea" how many people had died, when asked about fatalities because of the inadequacy of the response.
"We're not into the blame game... I've been trying to save lives," she said.
The federal emergency management association has asked for patience, saying it will take time to reach people, given the magnitude of the disaster.
According to the White House, about 90,000 sq miles (234,000 sq km) has been affected by the hurricane.
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Originally Posted by JPaul
Mayor Ray C Nagin has emerged as the voice of New Orleans over the last week, first ordering the evacuation of the city in advance of Hurricane Katrina and now criticising the federal Government in a series of increasingly direct interviews.
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