Fifteen civilians were hurt when a hand grenade thrown at an army convoy in Pakistan's North Waziristan tribal area missed its target, officials said.
The grenade was thrown at a convoy travelling to a garrison at Bannu, near the main town of Miran Shah.
Government official Mohammed Jamil said at least two of the injured were in serious condition.
Pakistan stepped up military operations a year ago against suspected al-Qaeda and Taleban militants in the region.
The grenade reportedly landed on a private vehicle. The attacker escaped, officials said.
Attacks on troops in the region are fairly common. On Wednesday, five rockets landed close to an air base at Miran Shah but no one was hurt.
Pakistan has deployed about 70,000 troops to the Afghan border region in its operation against militants.
The army believes hundreds of militants, including Arabs, Afghans and Central Asians, are holed up there.
Also on Thursday, Pakistani security officials told the AFP news agency they had arrested six suspected al-Qaeda-linked foreigners in a raid in Peshawar, the capital of North-West Frontier Province.
An official said the material seized - grenades, a pistol, computer disks and militant literature - suggested links to al-Qaeda.