The top US general in eastern Afghanistan said he is seeing "some very interesting movement" of insurgents across the border into Pakistan this spring, possibly to join Taliban militants battling government troops.
Fighters from both nations have long moved back and forth across the porous frontier, a mountainous region that has been a sanctuary for both Al-Qaida and the Taliban.
But Maj Gen Jeffrey Schloesser's remarks to AP in an interview on Friday suggest a larger transfer into Pakistan than has been seen previously, as the fighting between Pakistan's troops and the Taliban has intensified.
There has been concern in Islamabad and Washington that the buildup of 21,000 additional US Forces in Afghanistan may push Taliban militants into Pakistan, further destabilizing the border region there.
The Obama administration has declared eliminating militant havens in Pakistan as vital to its goals of defeating Al-Qaida and winning the war in Afghanistan.
Schloesser suggested that most of the movement in the past has been from Pakistan into Afghanistan, calling the new development "an interesting movement backward".
He did not provide details or numbers of those heading toward Pakistan.
It is unclear to what extent the Taliban is moving to help militants in Pakistan or fleeing from US Forces in Afghanistan.