National Guard troops helping to guard borders with Canada and Mexico can't look for lawbreakers while they are flying to their new posts, and some haven't been allowed to carry weapons on duty. 

White House officials say such problems show why the Bush administration is studying whether to ask Congress to loosen the 124-year-old ban on soldiers doing law enforcement work inside the United States. 

``Let's review those kind of issues right now, when we have some time, so in the future if the military needs to be called up domestically, we have these issues worked out,'' said Gordon Johndroe, spokesman for the White House Office of Homeland Security. 

But Pentagon officials, including Defense Secretary Donald H. Rumsfeld, say there's no good reason to change the military policing ban, known as the Posse Comitatus Act. 

``It's not clear to me that there's any need to change Posse Comitatus at this time,'' Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said last week. ``It has not been pointed out what the advantages to doing that would be.'' 

Military officials have resisted any expansion of their domestic role, saying troops are trained to fight wars, not crimes. Added domestic duties since the terrorist attacks — such as providing fighter jet patrols over major cities — also have strained some military units' time and budgets. 

One top military leader said, however, he would support an expanded role for the military inside the United States. 

Air Force Gen. Ralph E. Eberhart, who will head the new military command charged with defending American territory, told The New York Times he would support changes in Posse Comitatus that would help the military protect Americans. He did not say what specific changes he favored. 

Rumsfeld said he didn't plan to recommend any adjustments to the military's role. ``I don't think anyone should hold their breaths waiting for changes in Posse Comitatus,'' he said. 

Congress passed the law in 1878, at the end of the Reconstruction period after the Civil War. It ended the Army's police role both in the South and in the West, where soldiers either enforced the law directly or local officials pressed them into service as a posse. 

Posse comitatus is a Latin phrase meaning ``power of the county.'' In this case, it's another way of saying law enforcement is not the military's responsibility. 

``We don't want a military dictatorship, and this is one of the ways of preventing that,'' said Michael Spak, a professor at Chicago-Kent College of Law and a former military lawyer. 

Congress has softened the law several times, allowing the military to offer equipment and support to fight drug smuggling and respond to terrorist attacks using chemical, biological or nuclear weapons. 

Military personnel still cannot make arrests within the United States, however. 

The law does not apply to National Guard units when they are acting under the authority of the states. Guard units often help respond to riots, natural disasters and other crises. 

When the president calls the National Guard into federal service, however, the Posse Comitatus Act does apply. That's why President Bush asked state governors to call up Guard units to provide airport security after the Sept. 11 attacks, spokesman Johndroe said. 

``This took the lawyers several days to decide,'' he said. ``It's one example of something that needs to be reviewed. ... We have a situation where we need to deploy troops, but we have to talk to a lawyer to figure out if we can do it or not.'' 

Homeland security chief Tom Ridge has said giving the military the power to make arrests was ``very unlikely.'' Top Senate Democrats also have said they would oppose giving troops that much power. 

``I don't fear looking at it to see whether or not our military can be more helpful in a very supportive and assisting role — providing equipment, providing training, those kind of things,'' said Senate Armed Services Committee Chairman Carl Levin, D-Mich.