The blast, which went off as the convoy passed through the Swat valley, hit a military truck laden with explosives, triggering a massive explosion.
It remains unclear whether the attack was carried out by a suicide bomber, or whether a roadside bomb trap was set.
On Wednesday, around 2,500 troops were sent to the valley, following a militant bomb attack that killed four troops late on Tuesday.
Major General Waheed Arshad said the troops would perform a large-scale operation against several thousand militants led by Maulana Fazlullah, the leader of Tehrik Nifaz-e-Sharia Mohammed, a pro-Taliban militant group.
The Swat valley was until recently popular with tourists for its historical Buddhist sites. However, the area saw an increase in militant activity after Maulana Fazlullah, speaking on an illegal FM radio station in July, declared jihad against Pakistani authorities. Islamist extremists have also burnt TV sets, computers and video equipment, local media reported.
There are also reports that Islamists have partially destroyed a seventh-century statue of Buddha in the valley.
