The attack sent shockwaves across the world, and has been widely condemned.
Pakistan announced the arrest of 12 members of the Pakistani Taliban over their alleged involvement in a deadly attack on a Pakistani school, an army spokesman said.
Another nine members of what is believed to be a 27-member cell have already been killed, Major-General Asim Saleem Bajwa told a media briefing on Thursday.
Bajwa credited the cooperation by neighbouring Afghanistan – where six of the men were arrested based on tips from Pakistani intelligence.
“Our cooperation is growing,” Bajwa said – particularly since the December 16 assault on an army-run school in the city of Peshawar that killed 150 people, mostly uniformed schoolchildren.
In the wake of that attack, both Pakistan and Afghanistan mutually pledged to work together and fight armed groups.
Previously the two neighbours routinely traded accusations that each was harbouring the other’s fugitives in lawless tribal areas along their mutual border.
The Taliban are a loose umbrella of dozens of local armed groups bent on toppling the Pakistani government and installing their own harsh brand of Islamic governance.
Taliban attacks have killed tens of thousands of Pakistanis in the past decade.
Bajwa, the army spokesman, said Pakistan has also been working closely with the Afghan government over the hunt for Pakistani Taliban chief Mullah Fazlullah – who is believed to be hiding in Afghanistan.
He showed taped confessions from two of the arrested fighters, who said Fazlullah ordered the attack and assigned commanders.
He also identified a mosque prayer leader who sheltered the attackers the night before they stormed the school.
Bajwa claimed that an ongoing Pakistani army offensive launched last June against fighters in the North Waziristan tribal region was progressing well, saying that the fighters had been squeezed into a corner.
But he sought international community support for the ongoing fight against the fighters.
“I want to say that it is time for the world to do more for Pakistan,” Bajwa said.