The massive effort to deploy and maintain U.S. troops and weapons in Saudi Arabia is taxing the military's support systems so severely that the services may seek a controversial presidential activation of reserve forces to sustain the operation and maintain other military missions worldwide, according to Pentagon officials.

Defense Secretary Richard B. Cheney, faced with a military that in recent years has increasingly tied its ability to maintain lengthy operations to its reserve forces, is now weighing whether to ask President Bush to order the first mobilization from civilian life of reserve soldiers, sailors and air personnel in a crisis since the Vietnam War, officials said.

Already the Joint Chiefs of Staff have reqested thousands of volunteers from National Guard and reserve units in more than 20 states. Military officials say that thus far, they have received more volunteers than actually needed for the air refueling operations, communications and other services where reservists are needed.

"The calling up of the reserves is something that is very much under consideration -- it has to be," said one ranking Pentagon official.

In the past decade, the military has assigned large portions of its combat support efforts to the reserves, with about 70 percent of the Army's medical, intelligence and logistical support needs now handled by the reserves.

But proposals for a presidential call-up of guard and reserves units raise sensitive political questions, according to several administration and military officials.

"When you call up the reserves, there is a real political consideration," said one military official. "While everybody may be rah-rahing the president's decision to commit troops, when you yourself have to go, it may not be quite as attractive."

The military has a National Guard and reserve force of about 1.6 million men and women, compared to the 2.1 million active-duty forces. Most guard units are assigned combat roles, while most reservists serve in support units.

Under federal law, the president can order the activation of up to 200,000 guard and reserve troops for 90 days, with the option of extending the call-up for another 90 days. Longer and larger activations require congressional approval.

Military leaders say they believe they would need far fewer than the full allowance of 200,000 reservists, officials said. Pentagon authorities said that while the services have received numerous individual responses to requests for guard and reserve help, the military soon may need to mobilize entire units, action that could not be taken without a presidential order.

In addition, most civilians serve in their guard and reserve jobs for limited periods ranging from two to several weeks because of the demands of their civilian jobs, officials said. If U.S. troops remain in Saudi Arabia for several months, the services would quickly deplete the availability of volunteers, officials said.

Military officials have said the number of all ground troops in Saudi Arabia could swell to 200,000 in the coming weeks.

Army combat troops, Air force fighter and bomber squadrons and Marines require massive support services, ranging from troops who serve the food to pilots who refuel the jets to communications and intelligence specialists. Much of that support is provided by reservists.

Pentagon officials noted that 61 percent of the Army's hospital personnel are reservists, 54 percent of its intelligence units and 44 percent of those who haul and store ammunition.

In the Marine Corps, almost two-thirds of the fuel supply units are in the reserves and the Air Force depends on reserve units for 35 percent of its tactical airlift, 67 percent of its aerial medical evacuation teams and 59 percent of the units that repair battle damage to aircraft, according to Pentagon officials.

But many combat units also include reserve components. The 24th Infantry Division is composed of two active-duty brigades and one National Guard brigade. While elements of the active-duty units are being dispatched to Saudi Arabia, the guard brigade could not be sent without presidential order.

The huge effort to move large numbers of forces to Saudi Arabia over a short period of time has forced the military to divert much of its support from other military operations in the United States and overseas. Military officials are now debating whether reserve and guard troops that could be activated by the president would be deployed to the Middle East or to military units in the United States and other parts of the world to fill in for troops headed to Saudi Arabia.

Congress has encouraged, and in some instances demanded, that the services pour more resources in the guard and reserves, partially in an effort to force a presidential call-up of civilian troops in the event of a large-scale operation.

Congressional supporters of those efforts say the law is designed to force the president to consider public support in any such decision in hopes of preventing a repeat of U.S. involvement in an unpopular Vietnam-type war.

Military officials said yesterday no consideration is being given to reinstituting an involuntary draft. However, 18-year-old males are required to register with the Selective Service system. Registration has traditionally exceeded 95 percent of all eligible males.