Kabul, Afghanistan ? Hundreds of Islamic fundamentalists in Pakistan’s tribal districts along the jagged eastern border with Afghanistan have taken up defensive positions along hilltops to defend against what they believe are imminent U.S. air strikes or ground assaults, Pakistani intelligence officials said Tuesday.

This unexpected mobilization of Taliban sympathizers in Pakistani territory raises the prospect that U.S. soldiers might find themselves fighting Pakistani citizens in Pakistan, compounding the political problems facing a government in Islamabad that’s pledged to support the U.S. war against terrorism.

Afghan villagers look at a field in Talak, eastern Afghanistan, where U.S. special forces found weapons left behind by Taliban forces. Taliban sympathizers in Pakistan expect U.S. troops to carry their search for Taliban and al-Qaida members across the border, and are arming to fight such an incursion.

It comes just days after Pakistani scouts, working with about a dozen U.S. commandos, raided an Islamic study center in the Pakistani border town of Miram Shah, seeking Jalaluddin Haqqani, a minister in Afghanistan’s ousted Taliban government who U.S. intelligence officials believe may have been sheltering terrorist leader Osama bin Laden. Haqqani was not captured, but the incident reportedly provoked intense local outrage.

U.S.- led special forces twice exchanged fire with suspected al-Qaida militants on Monday and the pre-dawns hours of Tuesday along the Afghan side of the same border area, killing four al-Qaida soldiers, according to U.S. Maj. Gen. Franklin L. Hagenbeck

“There’s been a sharp reaction from tribal areas which we had not been expecting,” one Pakistani intelligence official said Tuesday on condition of anonymity. “They are ready to take up arms and fight off American soldiers.”

Such a confrontation would pit Pakistan’s army against Islamic militants in the country and could provoke renewed protests against military strongman Gen. Pervez Musharraf.

Maj. Bryan Hilferty, a U.S. military spokesman at Bagram Air Base outside Kabul, said the firefight involving U.S. units took place less than 2 miles inside the Afghan border where U.S. forces had been staking out suspected Taliban soldiers. It was the first U.S. engagement with suspected al-Qaida and Taliban forces in nearly two weeks. Afghan officials predict a new offensive in the area within the next few weeks to root out remaining Taliban and al-Qaida forces.

One Pakistani intelligence official, who asked not to be identified, said Tuesday that the Islamabad government believed that as many 4,000 Taliban and al Qaida forces had moved into Pakistan, but that most of them had split up into small units. “We think they are dispersed and mainly in hiding.” He added that the Pakistanis did not believe Osama bin Laden had entered the country.

“They are expecting further raids and they intend to fight back,” he continued.