Pakistan is observing a day of national mourning in the wake of a militant attack on a university that claimed 21 lives in the restive Khyber Pakhtunkhwa province.

Flags were flying at half-mast on all government buildings inside and outside the country on Thursday while a prayer ceremony was being held in the capital, Islamabad.

The attack occurred on Wednesday when Taliban militants entered Bacha Khan University in Charsadda District and opened fire on students and teachers in classrooms and hostels.

Prime Minister Nawaz Sharif vowed a “ruthless” response to the massacre and ordered security forces to hunt those behind the attack.

Reports said schools in the area, 29 kilometers (18 miles) from the provincial capital of Peshawar, were open on Thursday.

“Militants want them shut down,” provincial Education Minister Arif Khan told AFP. “We wanted to send the message that education will continue.”

Amnesty International described the assault as a potential war crime.

The armed men who carried out the attack "violated the central principle of international humanitarian law by deliberately targeting civilians in what appears to be a war crime," the group said.  

“In its response to this latest tragedy, the Pakistani authorities must do everything in their power to protect civilians, while respecting human rights and international humanitarian law,” Amnesty said.

The incident took place more than a year after pro-Taliban militants stormed an army-run school in Peshawar and killed about 150 people, including 134 students.

Pakistan has been facing security challenges since it joined an alliance with the US in the so-called war on terror in 2001.

Thousands of people have been killed over the past years as a result of a surge in violence in the country.