"The United Nations stands ready to help with any kind of disaster expertise that might be required ... in full recognition that the United States is the country in the world that possesses the greatest civilian and military search and rescue and recovery assets themselves," Egeland told Reuters in an interview.

He said US officials had thanked the UN for its offer, but had not requested any assistance so far.

Egeland called Katrina one of "the largest, most destructive natural disasters ever."

Egeland said you could not compare the tsunami and Katrina in terms of the human toll, but in purely economic costs, this week's storm that ravaged New Orleans and other parts of the US Gulf coast could easily dwarf the devastation of other recent natural disasters.

While the UN estimated to tsunami's cost at $10 billion, estimates of insured losses from Katrina ranged from $9 billion to $25 billion on Tuesday.

Tens of thousands of homes and businesses have been destroyed, more than 78,000 people have been evacuated to emergency shelters, and the death toll has now reached at least 200.

Egeland, who had ruffled US feathers in December when he initially called rich nations' tsunami donations "stingy" before rowing back, said both the US government and aid organizations had limited even greater devastation from Katrina by evacuating citizens before the storm hit.

"The whole world is now watching the heart-wrenching scenes of how millions of Americans have had their lives affected, homes lost, livelihoods lost and there is of course an enormous wave of sympathy from all over the world," he said.