A day after an Islamic State group suicide bomber struck the crowded Sufi Shahbaz Qalandar shrine in Sehwan in Pakistan's southern Sindh province, killing nearly 100 people and leaving many others injured, Pakistani officials said that at least 39 suspected militants have been killed in the raids across the country.

The responsibility for the blast was claimed by ISIS through Amaq, a news agency affiliated to the Sunni militant group. The group said it had targeted a "Shiite gathering."

Thursday's attack was the deadliest in Pakistan so far this year. 

The Pakistani military said it had given a list of 76 suspected "terrorists" to Kabul, who they believe are hiding in Afghanistan. They have demanded an immediate action by Afghan authorities, and said the suspects should be handed over to Islamabad.

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

Pakistani Prime Minister Nawaz Sharif condemned the attack and vowed to track down the ones responsible for it. The Afghan president also condemned the attack.

"The explosion took place when a large number of people were inside the shrine boundary," a local police officer reportedly said. "A huge number of people come to the shrine every Thursday to take part in ritual dances and prayers. It is not possible to ensure the security of every person coming and going."

The bomber entered the main hall of the shrine through its golden gate and detonated an explosive near the site where the ritualistic Sufi dance "Dhamal" was taking place. "He first threw a grenade to cause panic and then blew himself up," Jamshoro Tariq Wilayat, a senior police officer, reportedly said.

"I saw bodies everywhere. I saw bodies of women and children," a witness, identified as Raja Somro, reportedly said.

Sughra Bibi, a 45-year-old woman who was rushed to a hospital with shrapnel wounds in her stomach, said she was near the front of the crowd watching the devotional dancing when the explosion took place. "The terrorists are targeting us just because they hate our shrines," she reportedly said. "They attacked another shrine a couple of months ago. But we will never give up our faith."

The U.S. State Department also said it would offer support to Pakistan.

"The United States strongly condemns today's terror attack in Pakistan against peaceful worshipers at one of the country's most revered Sufi shrines, which appears to have killed more than 70 people and injured scores more. We extend our condolences to the victims and their families and wish a full recovery for all of those injured. We also offer our support to the Government of Pakistan as it works to bring the perpetrators of this crime to justice," acting State Department spokesman Mark Toner said in a statement.