ISLAMABAD, Pakistan—Authorities in Pakistan said Tuesday they were trying to determine whether Islamic State was behind a suicide bombing that killed at least 72 people the day before—a development that could mark a new and bloody phase in the country’s battle with jihadist terror.
The militant group, which controls swaths of Syria and Iraq, said in a statement carried by its media arm on Monday that it was responsible for the assault. A faction of the Pakistani Taliban also claimed to have orchestrated the bombing, which targeted lawyers in the city of Quetta, in Balochistan province.
Investigators said it wasn’t immediately clear which group was the perpetrator or if the two had collaborated in the attack, in which a suicide bomber detonated explosives in a crowd of attorneys who had come to a hospital to collect the body of an assassinated colleague.
“Daesh can attack anywhere in the world, so why not in Pakistan?” said Anwaar ul Haq Kakar, a spokesman for the Balochistan government, using the Arabic acronym for Islamic State. “They have declared war globally on everyone.”
Islamic State, linked to multiple terror attacks in Europe this year, has sought to open a new front in South Asia. The group has a base in eastern Afghanistan, and security officials in India, Bangladesh and Pakistan say it has been seeking recruits in their countries.
Last month, Islamic State said it was responsible for an attack on a restaurant in Dhaka, the Bangladeshi capital, which killed 22 people. Security forces killed five militants. Bangladeshi authorities blamed a domestic militant group, Jamaatul Mujahideen Bangladesh, for perpetrating that attack.
Some attacks in Pakistan have been claimed in the name of Islamic State since early last year, but Pakistani officials have insisted in the past that the perpetrators were inspired by rather than operationally connected with the group.
This time, the assertion came from the official Islamic State media arm, which is considered reliable. Unusually, the statement was issued in three languages—Arabic, English and Pakistan’s national language, Urdu—according to SITE Intel Group, which monitors the online activity of extremist organizations.
A militant who said he was working for Islamic State and lives near Pakistan’s border with Afghanistan told local reporters the group had been planning the Quetta carnage for two months and that they had targeted lawyers, who “support democracy and make laws.”
On Tuesday, Pakistan’s army chief, Gen. Raheel Sharif, ordered top commanders to use “full vigor to eliminate terrorists and sleeper cells”, according to a statement from the military.
Pakistan’s jihadist underworld, which includes violent sectarian groups, is potential fertile recruiting ground for Islamic State. But many Pakistani militants remain largely loyal to al Qaeda.
Security officials said initial findings showed the people behind Monday’s attacks had links with groups in Afghanistan.
Islamabad complains that the Pakistani Taliban and other militants have found sanctuary in eastern Afghanistan—a mirror of Afghanistan’s grievance that insurgents targeting Kabul are based in Pakistan. Both countries deny accusations of clandestine official support for those militant havens.
“Afghanistan doesn’t support terrorism but has suffered the most in the war on terror,” said Dawa Khan Minapal, a spokesman for the Afghan president.
Among the groups based in eastern Afghanistan is Jamaat-ul-Ahrar, the splinter group of the Pakistani Taliban that said it carried out Monday’s bombing in Quetta.
“Jamaat-ul-Ahrar is ideologically close to Islamic State but they haven’t accepted the caliphate,” said Muhammad Amir Rana, head of the Pakistan Institute of Peace Studies. Mr. Rana said he believes that so far, Islamic State is limited to scattered cells in Pakistan.
Afghan Taliban forces have fought those of Islamic State in Afghanistan in recent months, though there now signs of a truce between them.
–Qasim Nauman in Islamabad, Habib Khan Totakhil in Kabul and Syed Zain Al-Mahmood in Dhaka contributed to this article.