American forces are closely watching volatile eastern Afghanistan but have not yet figured out exactly how many Al Qaeda and Taliban are hiding there and what they're up to, defense officials said Tuesday.

They played down a report by Afghans that there have been recent sightings of Al Qaeda leader Usama bin Laden and top aide Ayman al-Zawahri in the area near the city of Khost.

"The Khost area is a tense situation. ... It remains a dangerous place," said Pentagon briefer Brig. Gen. John W. Rosa Jr. at a news conference. "I think it would be premature to start trying to characterize what we're seeing," Rosa told a news conference.

American forces have long focused on the region near the Pakistani border and believe pockets of Taliban or Al Qaeda are holed up there. But perceptions of what's happening shifts as the enemy moves and new intelligence is received, defense officials said.

The Khost area is a major land route into Pakistan to the east and borders where U.S.-led troops just conducted the largest land assault staged in the five-month-old campaign against Taliban and Al Qaeda forces.

The city itself also is saturated with weapons and has been the site of hostilities between competing Afghan warlords.

Rosa declined to answer questions about how many enemy may be in the area, how they are communicating and other questions about how they are operating.

"But it's always worth repeating, we expect and anticipate additional pockets of resistance," Pentagon spokeswoman Victoria Clarke said. "It is the MO of these people to try to regroup in some shape or fashion, so we fully expect it and that's one of the reasons we're still there."

Rosa said that while there had been no engagements with enemy fighters in over a week, coalition pilots flew 150 mission over Afghanistan Monday and continued to search the area of the recently completed Operation Anaconda — the large ground assault against other regroupings of Al Qaeda and Taliban early this month.

The flights included surveillance and reconnaissance planes and bombers on standby in case enemy forces are sighted.

Commanders have said they expect to encounter fighters in smaller groups and that activity will increase as spring weather emerges and troops on both sides can move around better.

Rosa said coalition forces could face extra problems with smaller enemy groupings.

"Obviously, you'd like to have them in one big cluster and be able to mount an attack and do as much damage as you can," he said. "When they get in smaller clusters, it makes it a bigger challenge to locate them, to track them, and each one of those small pockets, you have to develop a plan of attack. It makes it a little bit more intense from our perspective," he said.