Pakistan on Sunday executed four more militants after ending a six-year old moratorium on the death penalty following a Taliban school attack that killed 149 people, officials said.

The hangings follow those of two convicted militants on Friday after death warrants for the six men were signed the day before.

"Ghulam Sarwar, Rashid Tipu, Zubair Ahmed and Akhlaq Ahmed have been hanged for an assassination attempt on General Pervez Musharraf", Pakistan's former leader, a prison official in the eastern city of Faisalabad told AFP.

Pakistan's decision to reinstitute executions was slammed by human rights groups, with the United Nations also calling for it to reconsider the move.

Two senior government officials of central Punjab province, where the executions took place, confirmed the hangings.

"The brutal killers were clearly frightened and sought mercy from the jail staff on their cruel, inhuman and un-Islamic act," the state-run Associated Press of Pakistan (APP) said.

"They admitted that their brutal and inhuman acts had finally brought them to the gallows," it added.

The government ended the six-year ban on capital punishment for terror-related cases following Tuesday's bloody rampage on an army run school that killed mostly children.

Pakistan's military chief on Thursday signed the death warrants for the six militants who were on death row, after the government ended the moratorium on Wednesday.

On Friday two militants Aqil alias Doctor Usman and Arshad Mehmood were hanged in Faisalabad jail.

Aqil, who used the name Doctor Usman, was convicted for an attack on the army headquarters in Rawalpindi in 2009 and was arrested after being injured.

Arshad Mehmood was convicted for his involvement in a 2003 assassination attempt on former military ruler General Pervez Musharraf.

Pakistan's decision came as the country's political and military leaders have vowed to wipe out the homegrown Islamist insurgency following Tuesday's attack.

Pakistan imposed a de facto moratorium on civilian executions in 2008, though hanging remains on the statute book and judges continue to pass the death sentence.

Only one person has been executed since then, a soldier convicted by a court martial and hanged in November 2012.