ISLAMABAD: Pakistani authorities on Tuesday closed all the schools in the country's largest province, Punjab, following an alert over possible militant attacks, according to a government notice.

The warning comes a week after a breakaway Taliban faction attacked a northwestern university and killed 21 people, mostly students. That school — the Bacha Khan University in the northwestern town of Charsadda — reopened briefly on Monday but then closed indefinitely to give students more time to recover from the incident.

The government memo, which was obtained by The Associated Press, says there is intelligence that 13 Taliban fighters recently entered the country from neighboring Afghanistan and were planning suicide attacks on schools across Pakistan.

The schools in Punjab would remain closed till the end of the month, said the province's education minister, Rana Mashood Ahmad. He did not cite the alert but said the closures were due to harsh winter weather and heavy fog.

Schools were also closed in southwestern Baluchistan province for the usual winter break there. In the northwest and the south, schools remained open and it was not immediately clear if there where additional concerns that prompted the closures in Punjab.

The Charsadda attack revived memories of the horrific December 2014 Taliban attack on an army run school in the nearby city of Peshawar that killed 150 people, 144 of them children. The Bacha Khan University has also demanded a series of new security measures, including the extension of a perimeter fence, having a retired military officer take charge of the campus and getting gun licenses for teachers.

All four attackers who took part in the Charsadda assault were killed. Over the weekend, authorities announced the arrest of five others suspected of involvement in that attack.

Meanwhile, Pakistan's foreign ministry said it had summoned on Monday a senior Afghan diplomat, Syed Abdul Nasir Yousafi, to convey Islamabad's concern "regarding the use of Afghan soil by certain terrorist elements" involved in the Charsadda attack.

The Afghan diplomat was told that Pakistan's investigation has showed that handlers of the four attackers who stormed the Bacha Khan University "were operating from Afghan territory." Kabul was also asked to assist in bringing those individuals to justice.

There was no immediate response from Kabul officials.

All four attackers were killed in the shootout with Pakistani troops at the Charsadda campus.