US states stricken by Hurricane Katrina will take years to recover, President George W Bush said after he flew over devastated areas on the Gulf coast.
He vowed his cabinet will take over the aid operation for "one of the worst natural disasters" the US had seen.
An extra 10,000 troops are being sent to the worst-hit areas in the states of Louisiana and Mississippi.
New Orleans is to be fully evacuated, amid fears thousands may have died there as flood waters swept the city.
With most of the low-lying city now submerged, its remaining residents have no electricity and are running out of fresh water and food.
Plans have been announced to evacuate tens of thousands of people from New Orleans - including some 20,000 sheltering in its crowded Superdome stadium.
Army helicopters have been helping survivors to safety
Asked how many had died in the city, Mayor Ray Nagin said "Minimum, hundreds. Most likely, thousands."
BBC correspondent Alastair Leithead says widespread looting and the failure to stop water pouring in from burst embankments have added to the panic in the city, and most people are now desperate to leave.
Hundreds of soldiers and police have been diverted from rescue work to law enforcement duties, amid reports that heavily-armed gangs are ransacking the city.
Louisiana Governor Kathleen Blanco said she was angered to see the crisis bringing out the worst in people.
President Bush's aircraft circled low over the stricken region on Wednesday as it flew him to Washington, ending his month-long break in Texas a few days earlier than scheduled.
As he passed over towns whose rooftops alone remained visible above flood waters, Mr Bush said: "It's devastating."
"It's got to be doubly devastating on the ground."
Later addressing the nation from the White House, the president said the relief effort would initially focus on restoring power, communication and transport links.
He also announced the release of fuel from federal reserves to stabilise oil output, severely dented by hurricane damage to rigs in the Gulf of Mexico.
"This will help take some pressure off the gas price," Mr Bush said.
With conditions still deteriorating, the government has declared a public health emergency along the whole of the Gulf coast to speed up the delivery of food, water and fuel to the region.
Homeland Security Secretary Michael Chertoff said 1,700 truckloads of essential supplies were on their way there.
Medical shelters are being set up offering 10,000 beds, while the US military is providing dozens of rescue helicopters and boats.
The Pentagon has ordered 10,000 extra national guardsmen to be despatched to Louisiana and Mississippi.
This will bring to 21,000 the total number of troops in areas hit by the hurricane, including Alabama and Florida.
President Bush flew over areas worst hit by the hurricane
Officials in Mississippi state, to the east of Louisiana, have warned the death toll is likely to climb above the current 110.
The state's Harrison County bore the brunt of Hurricane Katrina as it slammed into Biloxi and Gulfport before heading inland.
Democratic senator Frank Lautenberg has accused the Bush administration of taking too long to respond.
"We are watching this devastation unfold on our televisions for days and you have to ask: where is the federal government?" the Reuters news agency quotes him as saying.
"We should have had a significant amount of troops and supplies there on the ground Monday."
Anger has also been reported among some of the poorer people hit hard by the hurricane.
"Many people didn't have the financial means to get out," Alan LeBreton, a Biloxi resident told Reuters news agency.
"That's a crime and people are angry about it," he said.