Pakistan's army spokesman has made clear that its forces will not tolerate incursions into Pakistani territory.
His remarks come a day after Pakistani troops were reported to have fired shots into the air to stop US troops crossing into South Waziristan.
Tension is rising in Pakistan over an increase in US attacks along the border with Afghanistan.
America's top military commander, Adm Michael Mullen, is in Pakistan on an unannounced visit, the Pentagon says.
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Meanwhile, British Justice Secretary Jack Straw has said operations along the border should be done "in a way that is not counterproductive".
"We believe that these matters have to be subject to consent of the Pakistan government because Pakistan is a sovereign nation," he told the BBC after talks in Islamabad.
On Monday, there were reports of nine US helicopters landing on the Afghan side of the border and US troops attempting to cross into South Waziristan.
The tribal region is one of the main areas from which Islamist militants launch attacks into Afghanistan.
Locals said 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.
Pakistan's military confirmed firing but denied that Pakistani troops were involved.
Army spokesman Major General Athar Abbas told the Associated Press on Tuesday that "no incursion is to be tolerated".
But he stressed to the BBC that no specific orders had been given to open fire if US troops crossed the border from Afghanistan.
Diplomatic fury
It emerged last week that US President George W Bush has in recent months authorised military raids against militants inside Pakistan without prior approval from Islamabad.
The BBC's Barbara Plett in Islamabad says there is a growing American conviction that Pakistan is either unwilling or unable to eliminate militant sanctuaries in its border area.
There have been a number of missile attacks aimed at militants in Pakistan territory in recent weeks.
Pakistan reacted with diplomatic fury when US helicopters landed troops in South Waziristan on 3 September. It was the first ground assault by US troops in Pakistan.
Locals in the Musa Nikeh area said American soldiers attacked a target with gunfire and bombs, and said women and children were among some 20 civilians who died in the attack.
On Monday, the tribesmen say they grabbed their guns and took up defensive positions after placing their women and children out of harm's way.
Pakistan's army has warned that the aggressive US policy will widen the insurgency by uniting the tribesmen with the Taleban.
(BBC)
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