Defense Secretary Richard B. Cheney told the military services yesterday that they can activate up to 49,703 National Guard and reserve forces personnel between now and Oct. 1 to support the U.S. deployment to the Persian Gulf.

As the mechanics of the reserve call-up were being detailed at the Pentagon, the chairman of the Joint Chiefs of Staff, Gen. Colin L. Powell, warned Iraqi President Saddam Hussein, "Don't try to scare us or threaten us. It won't work, never has."

Powell, speaking to the Veterans of Foreign Wars convention in Baltimore that President Bush addressed earlier this week, said, "No Iraqi leader should think for a moment that we don't have the will or ability to accomplish what might be required of us."

In Washington, Pentagon spokesman Pete Williams said 9,827 reservists will be called over the next nine days and another 36,876 are likely to be called up in September. Williams said the majority of those activated will be dispatched to the Persian Gulf.

The Marine Corps received authority to mobilize 3,000 reservists, but Williams said it was unlikely that the Marines would use the authority since the service depends the least on reserve forces to support combat troops.

Some reserve forces are needed urgently, Williams said. For example, he said, water purification experts are needed in the desert environment, where the massive military operation is straining Saudi Arabia's desalinization plants.

In addition, he noted, Air Force transport pilots are flying back-to-back flights between the East Coast and the Arabian peninsula, a 14,000-mile round trip. Williams told reporters at a noon briefing that 123 U.S. aircraft had landed in Saudi Arabia in the preceding 24 hours.

"The problem is," Williams said, "that they are simply flying so many flights that the Air Force pilots have reached the maximum number of hours that it's recommended that they fly per month for safety reasons." Williams said the call-up, the first summons for an emergency from the 1.16-million-member reserve force since 1968 and the largest since the Berlin crisis of 1961, will last for 90 days and can be renewed for another 90 days without consent of Congress.

"We have the authority to use their services for 180 days," Williams said, "and beyond that, I just can't make any time commitment."

A military official said yesterday that during a long meeting between Cheney and the military chiefs yesterday morning, the secretary emphasized President Bush's desire to minimize the reserve call-up and its potential political impact. Cheney has the authority to tap up to 200,000 members of the National Guard and reserves and the Army's proposal for an initial call-up would have activated up to 80,000 reservists.

"The White House wants the lowest profile on this they can get," the official said, pointing out that the call-up could more than double if hostilities were to break out between U.S. forces and the 160,000 Iraqi troops occupying Kuwait.

Williams said the identity of the the reserve units called up will be made public after unit members are notified through the military chain of command, beginning today.

Also yesterday, Cheney granted military chiefs the power to prevent personnel with needed skills to leave service during the period of the call-up.

Williams said this authority would allow commanders in the Persian Gulf "to keep critical skills in the theater of operation by delaying the departure of service members who are about to retire or have reached the end of their enlistment or service contract."

The mobilization will affect National Guard and reserve forces. The Army can summon 24,734 between now and Oct. 1; the Navy 6,243, and the Air Force 14,476 during the period. The call-up also includes 1,250 members of the Coast Guard, Williams said.

Also yesterday, Williams said the Navy has boarded "a few" commercial ships as part of the Persian Gulf "intercept" operation to stop all imports and exports from Iraq and occupied Kuwait. "I don't think any of them were Iraqi" ships, he added.

Williams declined to be more specific, adding that he was not aware of any new incidents of U.S. firing warning shots across the bow of tankers that refused to heave-to for search or interrogation. The first such shots were fired last Saturday, but the ships involved, both Iraqi-registered, refused to yield and proceeded on their way.

One ship, the Khanaqin, steamed to the Yemeni port of Aden and unloaded a portion of its cargo Tuesday, Williams said. "The Yemenis have assured the United States that the off-loading stopped before it was completed," he said.