Pakistani troops have fired shots into the air to stop US troopscrossing into the South Waziristan region of Pakistan, local officialssay.
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 increasingly aggressive US attacks along the border.
The latest confrontation began at around midnight, local people say.
It emerged last week that US President George W Bush has in recentmonths authorized military raids against militants inside Pakistanwithout prior approval from Islamabad.
The BBC’s Barbara Plett in Islamabad says there is a growingAmerican conviction that Pakistan is either unwilling or unable toeliminate militant sanctuaries in its border area.