Major-General Waheed Arshad, a Pakistani army spokesman, said the army did not carry out any operations in the area on Tuesday.
He said: "A group of some militants were making explosives and there was an explosion."
Arshad was unable to confirm casualties.
Pakistani military spokesmen have offered such explanations in the past when US forces in Afghanistan launched strikes on targets in Pakistani territory to avoid admitting any violation of sovereignty.
Last September, the Pakistan government struck a peace deal with tribal leaders in the region, under which foreign fighters were bound to either surrender or be expelled from North Waziristan.