The top U.S. general in eastern Afghanistan said Friday 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 Qaeda and the Taliban.

But Maj. Gen. Jeffrey Schloesser's remarks in an interview with The Associated Press 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 U.S. 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 vital to its goals of defeating Al Qaeda 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 U.S. forces in Afghanistan. Schloesser suggested that both factors could be at play.

At the Pentagon, a senior U.S. military official on Friday cited concerns by Pakistani military chief Gen. Ashfaq Kayani that the U.S. troop buildup in Afghanistan has been pushing the Taliban into Pakistan over the last several months.

But the official, who spoke on condition of anonymity to talk more candidly about the issue, downplayed the urgency of the situation and said there's plenty of time to plan for and deal with any Taliban migration if it comes.

A second official said there's no intelligence or evidence to indicate a specific strategy by the Taliban to move back into Pakistan as a result of the recent fighting in the nation's northwest territories.

Most Taliban safe havens are in Pakistan, and U.S. officials have for months pointed to its border with Afghanistan and Pakistan as a place where extremists have been able to move freely.

Schloesser, who commands American troops in eastern Afghanistan, suggested that some of the current movement may be intended to reinforce Taliban fighters in Pakistan.

"I would suppose that ... some of that movement is fighters going back to help their insurgent groups that are involved in fighting, for example in Bajur or the fighting that is occurring in Buner or in the Dir area or potentially even in Swat," Schloesser said.

Pakistani troops launched an offensive last month in the Swat region against militants who had pushed into the adjacent Buner district within 60 miles of the capital, Islamabad.

The army claims it has killed more than 1,000 militants and won back swaths of territory in Swat. But it faces stiff resistance. Earlier this year, Pakistan launched an offensive in the Bajur tribal area.

Pakistani military officers say Afghan, Tajik and Uzbek fighters are taking part in the current fighting in Pakistan's Swat Valley and in other border regions, but that the vast majority are Pakistani.

Schloesser's troops helped the Pakistani offensive by trying to prevent militants from crossing from Afghanistan into Bajur.

The area under Schloesser's command includes the provinces of Nuristan, Kunar, Khost and Paktika, all with active insurgent groups, some supported from within Pakistan. It abuts most of Pakistan's volatile tribal areas.

The current movement of fighters into Pakistan could also partly be a result of pressure from the thousands of new U.S. troops that have joined the fight in Afghanistan this year, Schloesser said.

In Washington, Joint Chiefs of Staff Chairman Adm. Mike Mullen said Thursday he was concerned that the U.S. troop buildup to roust insurgents from Afghanistan could further destabilize Pakistan.

However, Mullen, speaking to the Senate Foreign Relations Committee, said the military planning is under way to try to avoid that.

Mullen said he believes the upcoming increase of 21,000 U.S. forces in Afghanistan "is about right" for the new strategy of trying to quell the insurgency and speed up training of Afghan security forces.