BBC correspondent in Islamabad
The Pakistan military has launched a major operation against suspected al-Qaeda militants in the country's tribal region that borders Afghanistan.
The army carried out two operations in the area last year
A military spokesman said helicopter gunships were supporting ground troops.
The offensive is in the area of South Waziristan, where a number of fire fights with suspected Islamic militants have taken place in the past.
The area has been a hub of activity for members of Afghanistan's ousted Taleban regime and Osama Bin Laden's al-Qaeda.
The military crackdown started in the early hours of Thursday morning following intelligence reports that some suspected foreign militants were hiding in the mountains near Waziristan's main town of Wana.
Initial reports said that as the ground troops moved into the area to round up the suspects, they came under fire.
Helicopter gunships were called in to provide the air cover.
There has been no word on any casualties, but some houses have been destroyed in the assault.
None of the suspected militants have been found but some local tribesmen had been detained, a Pakistani army spokesman said.
South Waziristan, which borders Afghanistan's troubled Paktika province, has remained a hub of al-Qaeda and Taleban activity for quite some time.
Last year, the Pakistani military carried out two major operations in the area.
The fiercest one was in October, when eight suspected militants and two troops were killed.
One of the people killed was later identified as Hasan Mahsum, a leader of a Chinese Islamic militant movement.
Since early last year, the Pakistani military authorities have kept the entire tribal region of South Waziristan under siege, as there are suspicions that some high-ranking al-Qaeda militants may be hiding in the area.