At least 70 people died Thursday when a bomb ripped through a crowded Sufi shrine in Pakistan, police said, in the deadliest attack to hit the militancy-plagued country so far in 2017.

The Islamic State (IS) militant group claimed the attack in a statement carried by the Amaq propaganda agency linked to the Sunni militant group.

Pakistan's leaders vowed revenge for the attack on the shrine in the town of Sehwan in Sindh province, some 200 kilometres (124 miles) northeast of the provincial capital Karachi, which came after a bloody week of exremist assaults shook the country's growing sense of security.

A police source said a suicide bomber had entered the shrine of Lal Shahbaz Qalandar, a 13th century Muslim saint, and blown himself up among the devotees.

The shrine had been crowded on a Thursday, considered a sacred day for prayers.

"So far 70 people have been killed and more than 150 wounded," Inspector General of Police for Sindh province A.D. Khawaja told AFP.

"Many wounded people are in critical condition and they will be shifted to Karachi as soon as Navy helicopters and C-130 plane reach nearest airport."

Emergency services are basic in Sehwan, with the nearest main hospital some 130 kilometres away. Provincial health minister doctor Sikandar Maindro said an emergency had been declared in Karachi hospitals.

Pakistan's Prime Minister Nawaz Sharif swiftly issued a statement saying an attack on Sufis was considered a "direct threat".

Military chief General Qamer Javed Bajwa warned: "Each drop of (the) nation's blood shall be revenged, and revenged immediately. No more restraint for anyone."

Pakistan has seen a dramatic improvement in security in the past two years, but a series of attacks this week -- most claimed by the Pakistani Taliban -- has shaken the growing sense of optimism.

Jamaat-ul-Ahrar, a faction of the Pakistani Taliban said it had carried out a suicide bombing in Lahore which killed 13 people and wounded dozens more Monday, days after the group had vowed to launch a fresh offensive.

Four suicide bombers struck northwest Pakistan on Wednesday, killing six people and unnerving civilians further.

"The past few days have been hard, and my heart is with the victims. But we can't let these events divide us, or scare us," Sharif said in his statement.

"We have faced tougher circumstances, and still persevered."

The attacks underscore Pakistan's struggle to stamp out extremism, which was stepped up after the country's deadliest ever attack, a Pakistani Taliban assault on a school in Peshawar in 2014 which left more than 150 people dead -- mostly children.

The army intensified a long-awaited operation in the semi-autonomous tribal areas, where militants had previously operated with impunity, and the government launched a vaunted National Action Plan against extremism.

Emboldened Pakistanis are once again attending public gatherings and the growing confidence is palpable after more than a decade of militant attacks.

But critics have repeatedly warned that the crackdown does not address the root causes of extremism, and groups like the Pakistani Taliban -- and, increasingly, IS -- can still carry out spectacular assaults.