Four persons have been arrested in Pakistan for facilitating the deadly Taliban attack on a prestigious university in the country's northwest that killed 21 people, mostly students, the military announced today.

"We have arrested four facilitators who helped the attackers enter Pakistan and took them to Mardan before attack on Bacha Khan university in Charsadda district," Director General Inter Services Public Relations (ISPR) Lt Gen Asim Bajwa said.

On Wednesday, four heavily-armed terrorists attacked the Bacha Khan University named after the iconic Khan Abdul Ghaffar Khan in the volatile Khyber-Pakhtunkhwa province.

The attackers had entered Pakistan from Afghanistan via the Torkhum border, Bajwa said.

According to Bajwa, the main facilitator -- "terrorist A" -- who received and made arrangements for the attackers at Torkhum border checkpost is still at large.

The arrested men helped militants purchase their weapons in the country's tribal belt and relied on women close to the accused to help smuggle guns. "Women wearing traditional veils do not usually undergo a routine body search at the checkpoints," said Bajwa.

He said the attack on BKU was planned and controlled from Afghanistan as the phone call of commander Omer Mansoor, who later claimed responsibility, was made from Afghanistan.

Mansoor and his deputy Qari Zakir made calls to attackers from Afghanistan and on Wednesday - the day of the attack - around 10 calls were made from Afghanistan, he claimed.

He said that Pakistan army chief Gen Raheel Sharif has exchanged intelligence-based information with Afghan President Ashraf Ghani. Bajwa said one of the arrested men recently did masonry at BKU and later drafted a map to aid the attackers.

On Friday, the Taliban faction behind the massacre posted a video message vowing to target schools throughout Pakistan, calling them "nurseries" for people who challenge Allah's law.

The BKU assault came about an year after terrorists attacked an army-run school in Peshawar that killed nearly 150 people, most of them students. The Pakistani military intensified an ongoing offensive, named operation Zarb-e-Azb, against extremists in the tribal areas after the 2014 attack.

Bajwa said the operation was progressing successfully. To a question on border management with Afghanistan, Bajwa said the issue is under consideration and a lot of measures have been taken in this direction.