For the first time since the 1991 Gulf War, President Bush has approved the mobilization of thousands of U.S. military reserve forces. The move comes in response to Tuesday's terrorist attacks in New York and Washington.

The president has given Defense Secretary Donald Rumsfeld the authority to call up to 50,000 reservists to active duty.

But Pentagon officials say the armed forces, after reviewing their immediate requirements, plan to mobilize only about 35,000 - most of them, about 13,000, from the Air Force. The next largest contingent will be about 10,000 Army soldiers.

The reserve forces will be used primarily in what is called "homeland defense" - that is, security duties in the United States itself. The emphasis on Air Force reservists in the mobilization reflects the requirement for stepped up air patrols following Tuesday's multiple aircraft hijackings by suicide terrorists.

The reservists will also be used in recovery operations in New York and Washington.

Craig Duehring, the principal deputy defense secretary for reserve affairs, outlined the types of reserve personnel being called up in the coming days. "The kinds of units that might be called up include air defense, airlift, intelligence support, military police, medical, logistics, engineers, search and rescue, civil affairs, chaplains, and so forth," he said.

Those who are called up could remain on active duty for up to two years. But officials say they hope to keep duty terms shorter. They also say many reservists have been stepping forward to volunteer for duty.

Meanwhile, recovery operations are continuing at the Pentagon, where a hijacked American Airlines plane slammed into one of the building's five sides, triggering a massive fire and the collapse of tons of masonry, steel and other elements of the World War II era structure.

Pentagon officials estimate the death toll is between 190 and 250, including passengers who were on board the ill-fated plane.

They also estimate it will cost more than $100 million to repair the structural damage.