Officials said death toll from the deadly attack on Bacha Khan University in Charsadda district has reached 21.
Army spokesman Lt Gen. Asim Bajwa confirmed 20 students, teachers and security guards of the university were killed in the attack. One later died in hospital of wounds. Nearly 20 were also injured in the attack, the Xinhua News Agency reported Thursday.
The security forces killed all four terrorists after around three hours of clash.
A Pakistani Taliban commander, Omar Mansoor, has claimed responsibility; however, central spokesman for the group has denied any involvement.
The conflicting claims indicated further cracks within the ranks of the Taliban.
Prime Minister Nawaz Sharif declared Thursday as mourning day across the country. National flag was flying at half mast on all government buildings. The four provincial governments also announced mourning day.
The army spokesman told a late Wednesday press conference that the security forces have achieved a breakthrough to identify the attackers and those who planned the attack.
"One of the attackers was receiving phone calls on an Afghan mobile number," General Bajwa told reporters.
He also said the security forces have launched an intelligence-based operation, adding that action would also be taken against the financiers and facilitators of the terrorists.