Officials in the southern U.S. state of Louisiana say engineers have begun pumping floodwaters out of New Orleans, one week after Hurricane Katrina devastated the low-lying city.

The U.S. Army Corps of Engineers Monday completed sealing the main levee, which burst as a result of the storm. The engineers shored up the levee with metal sheets, and used helicopters to drop dozens of sandbags each weighing 12-hundred kilograms into the break to shut off the flow of water into the city.

Officials say it could take months to pump all the water out of New Orleans, which is under a mandatory evacuation order. Emergency crews are scouring the city to rescue survivors still trapped in their homes and collect the dead. Authorities say they fear the death toll could reach into the thousands.

Meanwhile, President Bush Monday made a second trip to areas in Louisiana and neighboring Mississippi ravaged by Katrina. The Bush administration has been heavily criticized for responding too slowly to the disaster.