The death toll from a massive Islamic State suicide bombing at a Sufi shrine in southern Pakistan has risen to 80, a senior government official has said.

The police commissioner in Hyderabad, the largest city closest to the town of Sehwan where the shrine was attacked, said a total of 250 other people were injured in the explosion.

Qazi Shahid Pervez said on Friday that local hospitals were overwhelmed and several of the critically wounded had been taken to Karachi, Pakistan's largest city on the Arabian Sea coast. He said military aircraft assisted in the evacuation of the casualties.

In Thursday's attack, a suicide bomber walked into the main hall at the Lal Shahbaz Qalandar shrine in Sehwan in Sindh province, and detonated his explosives among a crowd of worshippers.

IS claimed the attack in a statement circulated by its Aamaq news agency, saying it had targeted a "Shiite gathering".

The Sehwan shrine, which reveres a Muslim Sufi mystic, is frequented by the faithful of many sects of Islam but the majority of the worshippers are usually Shiite Muslims. IS reviles Shiites as heretics.

Overnight, Pakistani forces killed and arrested dozens of suspects in sweeping raids targeting militant hideouts. Shoot-outs with insurgents left at least 39 militants dead across Pakistan, according to three security officials.

Most of the operations were carried out by the paramilitary Rangers.

In one raid, the troops killed 11 suspects at a militant hideout in the port city of Karachi. In another, the Rangers came under fire as they were returning from Sehwan, and killed seven of the attackers.

Other raids took place in north-western Pakistan and also in the eastern province of Punjab. The officials said a total of 47 suspects were arrested in the raids.

Raja Somro, who witnessed the attack at the shrine, told a local TV network that hundreds of people were performing a spiritual dance known as the "dhamal" when the bomber struck.

"I saw bodies everywhere. I saw bodies of women and children," he said.

Local TV showed graphic footage of the aftermath of the blast, with wounded worshippers crying out for help and the floors covered with shoes, blood and body parts. Women cried and beat their chests in grief.

Prime Minister Nawaz Sharif vowed that security forces would track down the perpetrators of the attack, according to Pakistani state TV.

"Each drop of the nation's blood shall be avenged, and avenged immediately," Pakistan's army chief, General Qamar Javed Bajwa, said in a statement. "No more restraint for anyone."

The US State Department condemned the attack and offered its support to Pakistan in bringing the perpetrators to justice.

Thursday's attack was the deadliest in Pakistan following an assault on an army-run school in Peshawar on December 16 2014 that killed 154 people, mostly schoolchildren.

Pakistan has been at war with the Taliban and other extremist groups for more than a decade. In recent years it has launched major offensives against militant strongholds in the tribal regions along the border with Afghanistan, but insurgents have continued to carry out attacks elsewhere in the country.

IS has been expanding its presence in Pakistan in recent years and has claimed a number of deadly attacks, including a suicide bombing at another Sufi shrine in November 2016 in which more than 50 people were killed. A Taliban-linked group, Jamaat-ul-Ahrar, claimed responsibility for that attack.

The government has played down the IS affiliate, insisting that only a small number of militants have pledged allegiance to the group.

Afghanistan and Pakistan have long accused each other of failing to crack down on militants who operate along the porous border. Shortly after Thursday's attack, Pakistan closed the main Torkham border crossing with Afghanistan.

On Friday, Pakistan's military handed over to Kabul a list of 76 suspected "terrorists" allegedly hiding in Afghanistan, demanding they be captured and extradited to Islamabad.

A statement said the list was given to Afghan officials at the Pakistani army's sprawling headquarters in the garrison city of Rawalpindi.

The military did not specify who was on the list, but it has long claimed that the head of Pakistani Taliban, Mullah Fazlullah, and other militants are hiding on Afghan soil with the purpose of fomenting violence inside Pakistan.

Meanwhile, mourners gathered at the Sehwan shrine for a small rally to demand justice for the victims as some of the funerals got under way.

Relatives consoled the wailing mother of Zeeshan Ali, a 13-year-old who died in the blast and who was buried on Friday.

Zeeshan's uncle, Shoukat Ali, said he was devoted to his nephew and raised him since he had no children of his own.

"I raised him like my own child ... and they took him from me," he said.