QUETTA, Pakistan — Militants wearing suicide vests stormed a Pakistani police academy in the southwestern city of Quetta overnight, killing 61 people, mostly police cadets and recruits, and waging a ferocious gunbattle with troops that lasted into early hours Tuesday.

The four-hour siege — one of the deadliest attacks on Pakistan’s security forces in recent years — also wounded 123, mainly police trainees but also some paramilitary troops, according to Wasay Khan, a spokesman for the elite Frontier Corps. Some of the wounded were reported to be in critical condition.

The assault caught many of the recruits asleep in their dorms and forced cadets and trainers to jump off rooftops and run for their lives to escape the attackers.

Pakistani troops responding to the assault said it was over after all three suicide bombers involved in the attack were killed — one was gunned down while two others blew themselves up.

Later Tuesday, conflicting claims of responsibility emerged. The Islamic State group, which is waging war in Syria and Iraq where it has declared a self-styled caliphate, posted a claim on the group’s media arm, the Arabic-language Aamaq news agency. It said three IS fighters killed 60 police recruits in Quetta but the claim was not confirmed by Pakistani officials and IS did not offer any previously unknown details about the assault.

Earlier, a little-known breakaway faction of the Pakistani Taliban, known as the Hakimullah group, also issued a statement claiming responsibility for the attack. Pakistani officials, doubting the group’s capabilities in staging such a coordinated and spectacular assault, also could not confirm that claim.

While most of the casualties were cadets and others from the academy, some of the army personnel who responded to the assault were also among those killed, said Shahzada Farhat, police spokesman in Quetta, the capital of Baluchistan province.

The attack began at 11:30 p.m. on Monday, said Baluchistan Home Minister Sarfraz Bugti, with three militants shooting and killing a police guard at the watch tower before storming into the academy, located on the city’s outskirts.