President Bush says Hurricane Katrina is one of the worst natural disasters in U.S. history, and that the recovery operation will take years.

Mr. Bush gave the assessment at the White House Wednesday, hours after taking an aerial tour of devastated Gulf Coast areas on his way back from a vacation in Crawford, Texas.

The president said U.S. recovery efforts will focus on three priorities - saving lives, sustaining life in the affected areas, and rebuilding.

Mr. Bush said authorities are assisting New Orleans in evacuating any remaining residents. And he said federal authorities are working with local crews to help seal broken levees to halt flooding.

He said the federal government will transport more than five million meals and 13 million liters of water for victims in the hardest-hit regions.

The president said initial rebuilding efforts will include repairing bridges and roads, and restoring power. He said the challenges faced on the ground are unprecedented.

A White House spokesman said the president is likely to visit the flood-stricken areas later this week.