CHAKLALA AIR BASE, Pakistan -- Defense Secretary Donald H. Rumsfeld said he doubts that Osama bin Laden is in position to assert full command over the al-Qaida terrorist network.

Rumsfeld, who arrived Wednesday morning local time for an unannounced visit to Pakistan, said he found it interesting that bin Laden has not been heard from publicly in nearly a year.

"I don't know what it means," Rumsfeld told a group of reporters traveling with him. "I suspect that in any event, if he's alive and functioning that he's probably spending a major fraction of his time trying to avoid getting caught. I have trouble believing that he's able to operate sufficiently to be in a position of major command over a worldwide al-Qaida operation, but I could be wrong. We just don't know."

The defense chief also discussed a Pentagon announcement that U.S. troop levels in Afghanistan will drop by about 3,500 to roughly 16,500 next spring. He said the cancellation of a planned deployment there by a Louisiana-based brigade was an example of the way the Pentagon is likely to reduce the American troop presence in Iraq next year.

Rumsfeld and officials at the Pentagon said the 4th Brigade of the 10th Mountain Division, from Fort Polk, La., will stay home instead of going to Afghanistan as part of a normal 2006 troop rotation.

The deployment change was described earlier this month by Pentagon officials who had spoken on condition of anonymity because the plans were not finalized.

In an interview aboard an Air Force C-32 airplane carrying him from Washington to Pakistan, Rumsfeld said when U.S. commanders conclude that a smaller U.S. presence is advisable, some units scheduled to rotate into Iraq will have their tours canceled. At other times, units already in Iraq will be sent home early, he added.

Rumsfeld mentioned no specifics on future troop cuts in Iraq, beyond returning to a base figure of about 138,000 next month from the bulked-up 160,000-strong force assembled in advance of the Dec. 15 election.

The Pentagon hopes to drop the total well below 100,000 before the end of 2006. Those moves will depend on the strength of the insurgency, the progress in training Iraqi security forces and steps forward in building a national political consensus.

At this air base near the Pakistani capital of Islamabad, Rumsfeld was meeting with many of the approximately 850 U.S. troops providing humanitarian relief for victims of the Oct. 8 earthquake that devastated large parts of northwestern Pakistan and the disputed Kashmir region.

Rumsfeld said the United States has already begun scaling back the size of its military presence here, from a peak of more than 1,200 people several weeks ago. Aid efforts will continue for weeks.

The number of U.S. helicopters participating has dropped from 24 to 12, he said.