Pakistani troops have fired shots into the air to stop US troops crossing into the South Waziristan region of Pakistan, local officials say.
Reports say nine US helicopters landed on the Afghan side of the border and US troops then tried to cross the border.
South Waziristan is one of the main areas from which Islamist militants launch attacks into Afghanistan.
The incident comes amid growing anger in Pakistan over US attacks along the border region.
The confrontation began at around midnight, local people say.
They say seven US helicopter gunships and two troop-carrying Chinook helicopters landed in the Afghan province of Paktika near the Zohba mountain range.
US troops from the Chinooks then tried to cross the border. As they did so, Pakistani paramilitary soldiers at a checkpoint opened fire into the air and the US troops decided not to continue forward, local Pakistani officials say.
Reports say the firing lasted for several hours. Local people evacuated their homes and tribesmen took up defensive positions in the mountains.
The incident happened close to the town of Angoor Adda, some 30km (20 miles) from Wana, the main town of South Waziristan.
A Pakistani military spokesman in Islamabad confirmed that there was firing but denied that Pakistani troops were involved.