It is the first major military offensive ordered by Pakistan's new government
The Pakistani military has launched an offensive against militants near the main north-western city of Peshawar, security officials said.
Militants have become more active in and around Peshawar in recent months, say correspondents.
A contingent of troops has blocked the road towards Afghanistan, imposed a curfew and ordered shops to shut.
Pakistani militant leader Baitullah Mehsud said he was suspending peace talks with the government.
Troops have moved into the Khyber tribal region, close to Peshawar, with tanks and armoured vehicles.
Mortar rounds have been fired at suspected militant hide-outs in the mountains of the Khyber region, west of Peshawar.
"There has been no resistance, so far. No casualties, so far," Malik Naveed Khan, police chief of North West Frontier Province, told Reuters news agency.
Other officials said they thought most of the militants may have left the region ahead of the attack.
The Khyber Pass is one of the principal routes into Afghanistan from Pakistan and has long been a haven for smugglers and bandits.
After the attack began, Baitullah Mehsud told the BBC he was suspending talks with the government because security forces were targeting his forces in other regions.
The government has been in talks with him in an effort to pacify the tribal areas.
The militants the government is acting against are not part of the wider Taleban movement in Pakistan, but are still Islamists who wish to enforce their brand of Islam, says the BBC's Haroon Rashid in Islamabad.
The offensive is the first major military action the recently-elected government has taken against militants in the tribal areas near the border with Afghanistan.
When it took power two months ago, the government said it would negotiate with the tribes of the north-west to curb cross-border raids into Afghanistan and end the domestic militancy that caused havoc in Pakistan last year.
But now the government has authorised the army to back the talks with a credible threat of force.
The Afghan government and the coalition forces in Afghanistan have complained that Taleban militants are finding safe haven in Pakistan.
Increasing activity from militants around Peshawar lately has prompted the military to act.
Last week, 16 local Christians were briefly kidnapped from the heart of Peshawar, the capital of North West Frontier Province (NWFP).
In South Waziristan tribal region, where Baitullah Mehsud is based, 22 members of a tribe considered to be friendly to the government were killed after Taleban militants seized a town last week.