Intelligence agencies on Friday put Rawalpindi on high alert, said the military’s media wing, hours after a suicide bomber blew himself within the premises of a shrine in Sehwan Sharif.

“In wake of recent upsurge in terrorist incidents in the country, search and combing operations in areas of Rawalpindi district have been enhanced and security apparatus has been put on high alert,” ISPR said in a statement.

“Security forces and intelligence outfits have been instructed to further intensify combing and targeted operations with the aim to eliminate terrorists and sleepers cells.”

Authorities launched a nationwide security crackdown Friday, officials said, after the blast killed at least 70 people, including 20 children, and wounded hundreds.

Police had cordoned off the shrine of Lal Shahbaz Qalandar, a 13th century Muslim saint, early Friday as forensic investigators reached the town of Sehwan in Sindh, some 200 kilometres (124 miles) northeast of financial hub Karachi.

The centuries-old shrine's white floor was still smeared with blood, with scattered debris including shoes, shawls, and baby bottles. At least 20 children are believed to be among the dead, the head of Sehwan's medical facility Moeen Uddin Siddiqui said.

At 3:30am the shrine's caretaker stood among the carnage and defiantly rang its bell, a daily ritual that he vowed to continue, telling AFP he will "not bow down to terrorists".

The Islamic State group (IS) has claimed the attack, which came after a series of bloody extremist assaults this week, including a powerful Taliban suicide bomb in Lahore which killed 13 people and wounded dozens.

The attacks have dented growing optimism in security after Pakistan's decade-long war on militancy.

Military spokesman Asif Ghafoor said the attacks had been carried out from sanctuaries in Afghanistan and that Kabul had been asked to take action.

He also said the military had "closed" the porous border between the two countries where Kabul and Islamabad have long accused each other of harbouring extremists.

“Scores of suspects have been arrested from different cities in a pre-dawn crackdown,” a government official speaking anonymously told AFP.

Security officials said at least 18 terrorists had been killed in Sindh overnight, and 13 more in the northwest.

The provincial government announced three days of mourning as Pakistanis vented their grief and fury on social media.

Sufism, a mystic Islamic order that believes in living saints, worships through music and is viewed as heretical by some hardline groups.

The Islamic State group has targeted Sufi shrines in Pakistan previously, killing more than 50 at a shrine in Balochistan last year.