The Pakistani army is still searching for six militants it says helped carry out December's massacre at a school in Peshawar, in which 150 people died.
Major General Asim Bajwa said 27 were involved the attack, nine of whom had been killed and 12 captured. The dead are believed to include all of the gunmen who stormed the school.
He said Pakistani Taliban leader Mullah Fazlullah had masterminded the attack.
He is believed to be hiding in Afghanistan.
Gen Bajwa showed taped confessions from two of the arrested militants, who said Mullah Fazlullah ordered the attack and assigned commanders.
He said Pakistan was working closely with the Afghan government to find him.
Most of the plotters were Pakistani nationals who had planned the attack in the Afghanistan-Pakistan border area, he added.
So far, six suspects have been arrested in Afghanistan and six in Pakistan. All 12 were in Pakistani custody on Thursday. Six others remain at large.
"Afghanistan arrested these terrorists after Pakistan's army chief provided them with leads during his visit soon after the attack.
"The intelligence agencies of both countries worked together and we are very thankful to Afghanistan," he said.
One of suspects was named as Maulvi Abdus Salam, the imam of a mosque in Peshawar, whose "salary was paid by the government", said Gen Bajwa.
The detainees could face trial by military courts formed in the aftermath of the attack. The government has also reinstated the death penalty for those convicted of terrorism, with 22 people executed so far.
Since the massacre, teachers have been given permission to carry guns.
Security has been stepped up in the region, and a new combat group has been formed to tackle the Taliban.
Relatives of the killed children have held regular protests, demanding justice for their loved ones and more information about the attack.
Gen Bajwa claimed that an ongoing Pakistani army offensive launched last June against militants in the North Waziristan tribal region was progressing well and the insurgents had been squeezed into a corner.
However, he said more support was needed from the international community in the ongoing fight against the militants.
"I want to say that this is time for the world to do more for Pakistan,'' he added.
The group of attackers cut through a wire fence at Peshawar's Army School on 16 December before launching an attack on an auditorium where children were taking a lesson in first aid.
The gunmen, who were wearing bomb vests, then went from room to room shooting pupils and teachers in a siege that lasted eight hours.
Some 133 children and 17 adults were killed before the army retook control.
The army announced later that they had killed all of the militants involved in the attack.
A faction of the Pakistani Taliban loyal to Mullah Fazlullah said they carried out the attack in revenge for the army's offensive against them in North Waziristan.
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