US politicians have called for the head of America's disaster management agency to resign for failing to respond adequately to the New Orleans floods.

Michael Brown, the director of the Federal Emergency Management Agency (Fema), waited until five hours after Hurricane Katrina had struck before putting out an appeal for staff to help deal with the disaster, it emerged today.

In a memo to his boss Michael Chertoff, the Homeland Security Secretary, he proposed that the initial wave of 1,000 emergency workers would not be in place for two days, and that a further 1,000 would take a week to arrive. The delay was while staff were trained.

In his letter he describes Katrina - thought to have killed more than three times as many people as the 9/11 terror attacks - merely as a "near catastrophic event". The memo has now been leaked on the internet.

Before the storm, Fema had positioned front-line rescue and communications teams across the Gulf Coast. But officials acknowledged the first department-wide appeal for help came only as the storm raged.

On August 29, the day that the hurricane hit land, Mr Brown also warned fire and rescue services outside Louisiana, Mississippi and Alabama not to send in emergency workers unless specifically asked for help from local authorities.

In mitigation, a Homeland Security spokesman, Russ Knocke, said that Mr Brown's memo aimed to assemble a taskforce to co-ordinate with victims and community groups.

Instead of rescuing people or recovering bodies, these employees would focus on helping victims find the help they needed, he said.

"There will be plenty of time to assess what worked and what didn’t work," said Mr Knocke. "Clearly there will be time for blame to be assigned and to learn from some of the successful efforts."

Mr Knocke said the 48-hour period was to ensure workers had adequate training. "They were training to help the lifesavers."

In another part of his memo that has struck a jarring note with the US public, Mr Brown tells Fema staff that one of their duties is to make the agency look good.

"Convey a positive image of disaster operations to government officials, community organisations and the general public," he writes.

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