Authorities are trying to evacuate tens of thousands of people who remain in flood-swamped New Orleans, Louisiana - three days after Hurricane Katrina.

Reports from the city indicate an increasingly desperate situation, with storm refugees sleeping on streets and people growing weak from a lack of food and drinking water. Many people remain stranded on rooftops and in tall buildings waiting rescue.

A large evacuation effort continues at New Orleans' football stadium, the Superdome, despite reports of shots being fired and small fires being set in trash cans.

Police and National Guard troops have been deployed to stop looters who have been stealing not only food and water but also items like television sets and guns.

Widespread looting is also reported in the state of Mississippi, where coastal towns suffered heavy damage in Monday's storm.

President Bush said Thursday that the government is launching the most massive relief effort in U.S. history. The president plans to visit hurricane-ravaged areas on Friday.

Mr. Bush has asked former Presidents Bill Clinton and George Herbert Walker Bush to lead a fundraising effort for hurricane victims.

U.S. Senator Mary Landrieu of Louisiana said Thursday that the death toll from the storm could be in the thousands.