A series of bombings killed 115 people across Pakistan on Thursday, including 81 who died in twin blasts on a bustling billiards hall in a Shiite area of the southwestern city of Quetta.

Pakistan's minority Shiite Muslims have increasingly been targeted by radical Sunnis who consider them heretics, and a militant Sunni group claimed responsibility for Thursday's deadliest attack -- sending a suicide bomber into the packed pool hall and then detonating a car bomb five minutes later.

It was one of the deadliest days in recent years for a country that is no stranger to violence from radical Islamists, militant separatists and criminal gangs.

Violence has been especially intense in southwest Baluchistan province, where Quetta is the capital and the country's largest concentration of Shiites live. Many are ethnic Hazara who migrated from neighboring Afghanistan.

The billiards hall targeted Thursday was located in an area dominated by the minority sect. In addition to the 81 dead, more than 120 people were wounded in the double bombing, said police officer Zubair Mehmood. The dead included police officers, journalists and rescue workers who responded to the initial explosion.

Ghulam Abbas, a Shiite who lives about 150 yard from the billiards hall, said he was at home with his family when the first blast occurred. He was trying to decide whether to head to the scene when the second bomb went off.

"The second blast was a deafening one, and I fell down," he said. "I could hear cries and minutes later I saw ambulances taking the injured to the hospital."

Hospitals and a local mortuary were overwhelmed as the dead and wounded arrived throughout the evening. Weeping relatives gathered outside the emergency room at Quetta's Civil Hospital. Inside the morgue, bodies were laid out on the floor.

The bombs severely damaged the three-story building where the pool hall was located and set it on fire. It also damaged nearby shops, homes and offices.

Lashkar-e-Jhangvi, a Sunni militant group with strong ties to the Pakistani Taliban, claimed responsibility for the attack. Hazara Shiites, who migrated from Afghanistan more than a century ago, have been the targets of dozens of attacks by Lashkar-e-Jhangvi in Quetta over the past year, but Thursday's was by far the bloodiest.

Human Rights Watch sharply criticized the Pakistani government for not doing enough to crack down on the killings and protect the country's vulnerable Shiite community. It said more than 400 Shiites were killed in targeted attacks in Pakistan in 2012, including over 120 in Baluchistan.

"2012 was the bloodiest year for Pakistan's Shia community in living memory and if this latest attack is any indication, 2013 has started on an even more dismal note," said Ali Dayan Hasan, Pakistan director at Human Rights Watch.

"As Shia community members continue to be slaughtered in cold blood, the callousness and indifference of authorities offers a damning indictment of the state, its military and security agencies," Hasan said. "Pakistan's tolerance for religious extremists is not just destroying lives and alienating entire communities, it is destroying Pakistani society across the board."

Pakistan's intelligence agencies helped nurture Sunni militant groups like Lashkar-e-Jhangvi in the 1980s, to counter a perceived threat from neighboring Iran, which is mostly Shiite. Pakistan banned Lashkar-e-Jhangvi in 2001, but the group continues to operate fairly freely.

Earlier Thursday, a bomb targeting paramilitary soldiers in a commercial area in Quetta killed 12 people and wounded more than 40 others.

The bomb was concealed in a bag and placed near a vehicle carrying paramilitary soldiers, said Akbar Hussain Durrani, the provincial interior secretary. The bag was spotted by a local resident, but before the soldiers could react, it was detonated by remote control.

The United Baluch Army, a separatist group, claimed responsibility for the attack in calls to local journalists. Pakistan has faced a violent insurgency in Baluchistan for years from nationalists who demand greater autonomy and a larger share of the country's natural resources.

Elsewhere in Pakistan, a bomb in a crowded Sunni mosque in the northwest city of Mingora killed 22 people and wounded more than 70, said senior police officer Akhtar Hayyat.

No group claimed responsibility for that attack, but suspicion fell on the Pakistani Taliban, which has waged a bloody insurgency against the government in the Swat Valley, where Mingora is located, and other parts of the northwest.

Pakistan is also home to many enemies of the U.S. who Washington has frequently targeted with drone attacks. A U.S. missile strike in the northwest tribal region Thursday killed five suspected militants in the seventh such attack in two weeks, Pakistani intelligence officials said.

The recent spate of strikes has been one of the most intense in the past two years, a period in which political tensions between the U.S. and Pakistan led to a reduced number of attacks compared to 2010, when they were at their most frequent.

It's unclear whether the current uptick has been caused by particularly valuable intelligence obtained by the CIA, or whether the warming of relations between the two countries has made strikes less sensitive. Protests by the government and Islamic hard-liners have been noticeably muted.

The strike on Thursday occurred in a village near Mir Ali, one of the main towns in the North Waziristan tribal area, said Pakistani intelligence officials, speaking on condition of anonymity because they were not authorized to talk to the media.