A US troop build-up in Afghanistan could push Taliban fighters deeper into Pakistan, further destabilising it, the most senior US commander has warned, BBC reported.
Admiral Michael Mullen, who chairs the Joint Chiefs of Staff, was speaking at a meeting of the Senate Foreign Relations Committee in Washington.
Thousands of Pakistani troops are already battling Taliban militants in the north-west of the country.
Up to two million people in the area have fled the fighting, the UN says.
On Friday, the UN launched an appeal for $543m (£343m) to ease what it said was the "incredible suffering" of the internally displaced people in Pakistan.
In Washington, Adm Mullen told the Senate committee that the US had a clear national security interest in confronting the Taliban.
"They want Afghanistan back. We can't let them or their al-Qaeda cohorts have it. We can't permit the return of the... very same safe haven from which the attacks on 9/11 were planned and resourced," he said.
But Adm Mullen agreed with one of the senators that a US offensive in southern Afghanistan could force more Taliban fighters to flee into Pakistan.
"We can't deny that our success in that regard [in Afghanistan] may only push them [militants] deeper into Pakistan," he said.
"Can I... [be] 100% certain that won't destabilise Pakistan? I don't know the answer to that," he added.
However, Adm Mullen said US and Pakistani forces were planning measures to prevent this, without giving further details.
Adm Mullen's comments come as US President Barack Obama's administration prepares to send thousands of extra troops to Afghanistan.
With plans announced for a phased pull-out of US troops from Iraq, Afghanistan was recently confirmed as the primary focus of US military operations.
Mr Obama's new strategy is expected to pair non-military methods and reconstruction with a stronger armed force on the ground.