The homeless population of New Orleans since Hurricane Katrina has reached unprecedented levels for a U.S. city: one in 25 residents.

An estimated 12,000 homeless accounts for 4% of New Orleans' estimated population of 302,000, according to the homeless advocacy group UNITY of Greater New Orleans. The number is nearly double the pre-Katrina homeless count, the group says.

The New Orleans' rate is more than four times that of most U.S. cities, which have homeless populations of under 1%, said Michael Stoops, executive director of the Washington-based National Coalition for the Homeless. The cities with homeless rates closest to that of New Orleans are Atlanta (1.4%) and Washington (0.95%), he said.

A 2005 USA TODAY survey of 460 localities showed one in 400 Americans on average were homeless.

New Orleans Mayor Ray Nagin appealed to federal lawmakers this past week to provide funds and housing vouchers to help the city's homeless problem.

The percentage of New Orleans' homeless is one of the highest recorded since U.S. housing officials began tracking homelessness in the mid-1980s, said Dennis Culhane, a University of Pennsylvania professor who has studied homeless trends for more than 20 years.

"In a modern urban U.S. city, we've never seen it," he said of New Orleans' homeless rate.

Many of the homeless are Katrina evacuees who returned to unaffordable rents or who slipped through the cracks of the federal system designed to provide temporary housing after the storm, said Mike Miller, UNITY's director of supportive housing placement.

There are also out-of-state workers who came for the post-Katrina rebuilding boom but lost their jobs, and mentally ill residents in need of services and medication, he said. Many of the city's outreach homeless centers and public mental health services have been closed since Katrina.

Nagin has pledged to move the homeless from encampments around the city to more permanent shelters. Last year, the city and humanitarian groups found shelter for nearly all of the 250 people living in an encampment across from City Hall.

Nagin has suggested reinstating a city ordinance that would make it illegal to sleep in public places. Homeless advocates say the law would just crowd the jails.

"It just shows a real disconnect" between the city and the problem, said James Perry, head of the Greater New Orleans Fair Housing Action Center. "The answer is not going to be jails."