An extra 17,000 troops are being sent into the area of the southern US devastated by Hurricane Katrina.
President George W Bush said he had ordered in 7,000 front-line troops, and the Pentagon later announced it was deploying 10,000 National Guardsmen.
In a TV address, Mr Bush said he would not rest until he was satisfied with efforts to bring relief and order.
Mr Bush has been heavily criticised for his handling of the disaster, in which thousands are thought to have died.
In his address, the president admitted the response had been too slow, but said the sheer size of the disaster had hampered relief efforts.
The additional deployments will bring the number of regular and National Guard soldiers involved in the relief and rescue effort in the area up to 51,000.
Mr Bush said their priority would be to maintain law and order, and to assist in aid and evacuation efforts.
Large amounts of food, water and medicine have arrived in New Orleans, where many people - mainly from poorer black areas of the city - remain stranded five days after the hurricane struck.
Most of the tens of thousands who had sought refuge in harrowing conditions at the city's Superdome sports stadium and in the convention centre have now been to safety.
Most evacuees have gone to shelters in Houston in Texas and Baton Rouge in Louisiana.
But the head of the Federal Emergency Management Agency, Michael Brown, said there was "no humanly possible way of knowing at this stage how many people like that still exist in this vast urban area".
Mr Brown said relief workers had opened a mortuary and were collecting corpses, many of which have been floating down flooded streets.
And he warned looters and snipers in the city that they would soon be up against battle-hardened combat troops.
"Idiots with a gun on a rooftop" would not be allowed to derail the rescue drive, he said.
Most of New Orleans is under water and the US Army Corps said it might take months before floodwaters are pumped clear.
It is estimated that a million people have been left homeless across Louisiana, Mississippi and Alabama by Monday's hurricane and subsequent flooding.
Mr Bush usually pre-records his weekly radio address, and his decision to deliver Saturday's address live from the White House is being seen as a reflection of the pressure he is under.
He has cancelled a meeting with the Chinese President, Hu Jintao, and will visit the affected region again on Monday following his tour on Friday.
Mr Bush praised the efforts of relief and rescue workers, before adding: "Despite their best efforts, the magnitude of responding to a crisis over a disaster area that is larger than the size of Great Britain has created tremendous problems that have strained state and local capabilities.
"The result is that many of our citizens simply are not getting the help they need, especially in New Orleans. And that is unacceptable."
Black leaders have condemned the slow response to the disaster and civil rights campaigner Reverend Jesse Jackson suggested that racism was partly to blame.
"The reason one could argue that the rescue has been so slow is that some see us as foreigners." he said.
Some of Katrina's victims agree.
"If the victims were white, they'd be gone. They'd be sending in an army of helicopters, jets and boats," said Yvette Brown a black refugee from New Orleans.