Pakistani troops have started house-to-house searches for local and foreign Islamic militants in South Waziristan, near the border with Afghanistan.
By Zaffar Abbas
BBC correspondent in Islamabad
One of those they are seeking is Abdullah Mehsud, accused of being behind the kidnap of two Chinese men.
Local tribesmen are helping in the hunt for Mr Mehsud, a former inmate of the US military camp at Guantanamo Bay.
Officials say more than 400 people have been killed since the security operation began eight months ago.
The latest search operation is largely confined to one particular part of South Waziristan that is dominated by the Mehsud tribe.
Before moving into the area, security forces pounded the mountainous region for nearly two days, using heavy artillery and helicopter gunships.
Early on Saturday, ground troops moved in to start the house-to-house search with the help of local tribesmen.
Local customs forbid any outsider from invading the privacy of a home in this tribal region - officials say the operation has been made possible because the elders of the Mehsud tribe have agreed to co-operate.
The security forces are currently looking for Abdullah Mehsud, who is accused of masterminding the kidnapping of two Chinese engineers last week. One of the hostages died in a rescue attempt.
Mr Mehsud has also been accused of a number of other attacks against the security forces, and the authorities regard him as one of the most wanted Islamic militants in the area.
Mr Mehsud was arrested by US forces in Afghanistan nearly three years ago. But he was released from the US camp in Guantanamo Bay after 18 months as the Americans thought he was no more a threat to their interests.
Pakistani officials now say his arrest or elimination would help contain armed militancy in the tribal areas.
Since the security operation against the local and foreign militants was launched eight months ago, South Waziristan has been like a war zone. About 170 troops are said to be among the 400 killed in the fighting.