The vessels and thousands of National Guard and federal emergency specialists have been sent to Louisiana and Mississippi where hundreds of people are feared dead after Monday's storm.

"At this time, we are directing the full range of federal resources and capabilities to assist and protect those who have born the brunt of this catastrophe," said Homeland Security Secretary Michael Chertoff.

The first of six ships deployed is the USS Bataan, currently heading from Texas to Louisiana, which has a hospital and six helicopters on board to conduct emergency medical evacuations, according to US Northern Command (NorthCom).

The USS Iwo Jima, USS Shreveport, USS Tortuga and USNS Arctic are preparing to sail from Norfolk, Virginia with emergency equipment. The ships will be off the Louisiana coast in five days, said a Northcom statement.

The hospital ship USNS Comfort, leaving from Baltimore in the eastern state of Maryland, should arrive in seven days.

The Pentagon said it also plans to send the USS Grapple to help with "maritime and underwater survey and salvage operations".

Ten helicopters have also been dispatched to Louisiana and Mississippi to help Federal Emergency Management Agency (FEMA) operations.

Five are "capable of nighttime search-and-rescue and will also transport FEMA's Rapid Needs Assessment teams to gather critical information for state and federal emergency managers," the Pentagon said.

The military has also sent eight rescue teams with special flood expertise from California. Much of the low-lying city of New Orleans is under water after levees holding back Lake Pontchartrain broke and waters inundated more than 80 percent of the city.

The majority of the city's half a million residents have been evacuated.

Authorities have estimated that the death toll along the Gulf Coast is in the hundreds, as rescue crews confront extreme difficulties in trying to reach victims.

A state of emergency has been declared in storm battered areas in Louisiana, Mississippi, Florida, and Alabama.

The Homeland Security Department also gave details of a huge relief operation it is running:

-- FEMA has deployed 39 special disaster medical assistance teams from across the United States to the disaster zone.

-- Eighteen search and rescue task forces and other specialist incident teams have been sent to Louisiana and Mississippi.

-- FEMA is also moving emergency supplies -- such as water, emergency food rations, ice and equipment into the hardest hit areas. Tents and generators have also been sent.

-- The US Coast Guard has also rescued more than 1,250 people and 550 Coast Guard reservists have been recalled to active duty, the department said.

-- More than 31,500 National Guard members from the affected states are on emergency duty.

-- 250 mobile hospital beds with dozens of health specialists have arrived at Louisiana State University in Baton Rouge. Hundreds of other medical experts have been put on standby to go to the stricken zone.

The department said many key highways and bridges have suffered significant damage. Much of the I-10 road is clsoed in Louisiana and Mississippi is closed.

The American Red Cross is providing shelter for nearly 46,000 evacuees in more than 230 centres in Florida and across Alabama, Mississippi, Louisiana, Georgia, and Texas.

The Red Cross is launching the largest mobilization of resources for a single natural disaster involving thousands of trained workers, tons of supplies, and support.