About 30 soldiers from those captured had been freed recently after tribal peace committees, or jirgas, intervened.
The fighters started killing the soldiers after the Pakistani government's response to a suicide attack on a military convoy last week, a spokesman for the fighters said.
"The government has sabotaged the jirga process by launching an operation against innocent people," Zulfiqar Mahsud, the spokesman of Baitullah Mahsud, a local group commander, said.
"We will start beheading three soldiers every day and intensify attacks on security forces in different cities of the country if operations continue in the region."
The latest killings among the soldiers held captive came a day after Major-General Waheed Arshad of the Pakistani army said soldiers killed 10 fighters.
The opposition fighters were killed after they attacked an army checkpoint in Speen Wam, a border town in North Waziristan.
Fighters fired rockets at the same checkpoint Thursday, wounding three soldiers, officials said.
There have been a series of attacks by opposition fighters, many of them targeting security forces, since Pakistani troops raided the al-Qaeda-linked Lal Masjid (Red Mosque) in Islamabad in July.
Pakistan has said it will continue operations against local and foreign fighters.
The US has pressured Pakistan to increase operations against opposition fighters along the Afghanistan-Pakistan border amid concern that al-Qaeda fighters may be regrouping there.