Rumsfeld's pledge in congressional testimony echoed a report in The New York Times that the troops will be given the authority to shoot looters on sight.

"The forces there will be using muscle to see that the people who are trying to disrupt what is taking place in that city are stopped and either captured or killed," he said.

A Pentagon spokesman, however, said the report that looters will be shot on sight was "inaccurate."

"There has been no change in the rules of engagement," said Lieutenant Colonel David Lapan.

Rumsfeld said the deposed regime of Saddam Hussein had emptied Iraq's prisons, setting loose "looters, hooligans and bad people."

"They have to be rounded up and put back in. That takes a little time. You can't do that in five minutes," he said.

"There are also Baathists there. Not every one was captured or killed. And they don't wish us well. They still are part of the old regime. And they have to be rounded up and identified," he said.

The get-tough approach coincides with the arrival in Baghdad of a new civilian administrator, Ambassador Paul Bremer, who will be taking over from Jay Garner, a retired general who has been faulted for not establishing a functioning authority.

On the subject of weapons of mass destruction, Rumsfeld said there was some "intelligence chatter" indicating that some chemical or biological weapons may have been dispersed to other countries.

Senator Robert Byrd, a Democrat from West Virginia, asked why troops had not been assigned to secure known Iraqi nuclear sites if the goal of the US-led invasion was to prevent the spread of weapons of mass destruction.

"It is not possible to have enough forces in a country instantaneously to guard every site before somebody can get into it," Rumsfeld said.

Byrd said the United States had failed to heed warnings going into Baghdad of the lack of water and sanitation and medical supplies, and said, "We knew that military action would likely lead to mob action."

"At this point, there's little evidence that the US had in place any coherent plan for the reconstruction of Iraq following the end of combat," he said.

"I hope that the recent shake-up in the civilian leadership of the US occupation authority will help the situation and will not amount to merely rearranging the deck chairs on the Titanic," he said.

General Peter Pace, vice chairman of the Joint Chiefs of Staff, said additional troops from the 1st Armored Division were arriving in Iraq "as we speak."

General Tommy Franks, the commander of US forces in Iraq, was reviewing the situation "to see how they might reset themselves in the city to be able to provide the kind of patrolling and presence that is necessary to provide the stability they need," Pace said.

Currently, there are 142,000 US troops in Iraq, including 49,000 US troops in the greater Baghdad area. The arrival over the next few weeks of the 1st Armored Division will put another 20,000 troops at Franks's disposal, but the general has not yet decided what to do with them, Rumsfeld said.

He said he was told in a briefing earlier Wednesday that as much as three-quarters of the city is stable, but not "permissive."

"Another portion of it is less so, and most of the city at night, the hooligans are out and the criminals are trying to loot and do things," he said.

"We've had people shot and killed in the last 48 hours there, in Baghdad. It is a problem. The one thing that is central to success is security. We have a full-court press on that," he said.