US President George W. Bush said Thursday that the federal government has launched the most massive relief effort in its history to help the victims of Hurricane Katrina. He added that, for thousands more victims who still need to be rescued, help is on its way.

Bush, faced with his greatest emergency since the September 11 attacks, said he had no estimated death toll from the tragedy, except to say, "Obviously it's going to be a lot." Authorities in the Gulf Coast region have said the toll could be in the thousands.

Referring to insurance fraud and looting, Bush said there should be "zero tolerance" for breaking the law during an emergency situation.

Bush said he has not yet finalized plans to visit the devastated Gulf Coast. He expressed sympathy for those still stranded and acknowledged that thousands still need to be rescued.

"I just can't imagine waving a sign that says 'Come and get me now'," he said.

Bush brushed off criticism that he did not return to Washington from his month-long stay at his Texas ranch on Tuesday, in the immediate aftermath of the hurricane, instead of Wednesday.

"I hope people don't ... play politics during this period of time," he said.

Homeland Security Secretary Michael Chertoff, meanwhile, said one problem is that "we have an ongoing flood situation even as we're in the middle of recovering from the hurricane."

"We're in a position where there are additional people we have to look for," he said in an interview on NBC's "Today" show. "We're hoping to get as many people out as we can in the next 12 hours and 24 hours, but we're going to continue to search until we're sure we've got everybody safe."

The evacuation of the Superdome was suspended yesterday because of growing lawlessness outside the arena, as National Guardsmen in armored vehicles poured into New Orleans to help restore order across the increasingly desperate city.

An additional 10,000 National Guard troops from across the country were ordered into the Gulf Coast to shore up security, rescue and relief operations. That brought the number of troops dedicated to the effort to more than 28,000, in what may be the biggest military response to a natural disaster in US.

The US Army Corps of Engineers said floodwaters started to drop in New Orleans, which is mostly below sea level and was inundated by water from Lake Pontchartrain after levees broke.

Officials estimated it could take a month to get the water out.