Pakistani forces have launched a major operation in the border area close to Afghanistan.
Eyewitnesses report seeing as many as 25 military helicopters land at a little-used airport in Bannu district in North West Frontier Province, 40 kilometres (25 miles) from the border.
Officials say the exercise is routine, but it has fuelled rumours that an attack against suspected al-Qaeda fighters in Pakistan's lawless, semi-autonomous tribal areas may be imminent.
An army spokesman denied foreign troops were involved.
The US military has deployed an unknown number of special forces near the porous Afghan-Pakistan border in the hunt for al-Qaeda and Taleban remnants.
Al-Qaeda leader Osama bin Laden is among those who some think could be hiding there.
Pakistani troops were sent to the area in late June, the first time in over a century that the army had been deployed in tribal areas in the north-west.
Afghan leaders have recently criticised the authorities in Islamabad for not doing enough to stop cross-border attacks.