"There was a cluster of three houses and a tent which were hit. There were about 45 people in that area," a senior government official told Reuters.
A US drone aircraft carried out the attack at around 10:30 am (0530 GMT), according to intelligence officials and residents.
It was not known whether any leading Taliban or al Qaeda figures were targeted in the attack.
The Pakistani army had not carried out any operations in the area, Major-General Waheed Arshad, the army spokesman, said.
Initial reports suggested the explosion occurred while the fighters were making a bomb, he said.
Pakistani military spokesmen have offered such explanations in the past when US forces in Afghanistan have launched strikes on suspected targets in Pakistani territory to avoid admitting any violation of territorial sovereignty.