Four Pakistani intelligence officials and a senior local administrator told Reuters that the convoy was attacked on Saturday in Mir Ali area of North Waziristan district.
Around 10 other soldiers were wounded, some critically, and they were being flown to a military hospital, the sources said.
The office of the chief minister of Khyber Pakhtunkhwa province said it was a suicide bombing, adding it killed eight security officials.
Chief Minister Ali Amin Gandapur condemned the bombing and offered condolences to the soldiers' families.
"It was huge, a big bang," the local administrator told Reuters, adding that residents of the town could see a large amount of smoke billowing from the scene from a great distance.
One resident said that the explosion rattled the windowpanes of nearby houses, and caused some roofs to collapse.
No one has so far claimed responsibility.
The Pakistani military did not respond to a Reuters request for a comment.
The lawless district that sits next to Afghanistan has long served as a safe haven for different Islamist militant groups, who operate on both sides of the border.
Islamabad says the militants run training camps in Afghanistan to launch attacks inside Pakistan, a charge Kabul denies, saying the militancy is Pakistan's domestic issue.
Pakistani Taliban also known as Tehreek-e-Taliban Pakistan, an umbrella group of several Islamist militant groups, has long been waging a war against Pakistan in a bid to overthrow the government and replace it with its own Islamic system of governance.
The Pakistani military, which has launched several offensives against the militants, has mostly been their prime target.
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