A suicide attack here that left dozens of Shiite worshipers dead was apparently conducted by a militant group with a history of ties to Pakistan’s main intelligence service, a connection that threatened to escalate tensions in Afghanistan just as the United States plans its exit.

The bombing and a second attack on Shiites in northern Afghanistan killed at least 60 people, including a U.S. citizen, making Tuesday one of the deadliest days for civilians in the decade-long war. The strikes were highly unusual because they targeted members of Afghanistan’s Shiite minority, which was persecuted during the Taliban’s reign but which has not been a focus of insurgent bombings since the Taliban fell in 2001.

The Taliban denied any role in Tuesday’s attacks. But a spokesman for the Pakistani militant group Lashkar-i-Jhangvi called a station operated by Radio Free Europe to assert responsibility. If the claim is true, it would mark the first time that the group, which has ties to al-Qaeda, has carried out a major attack in Afghanistan.

Lashkar-i-Jhangvi militants have systematically assassinated Shiites and attacked their religious gatherings in Pakistan. If the group is extending operations into Afghanistan, it could add a highly destabilizing sectarian dimension to the costly and protracted Afghan war.

The attack could also worsen the already thorny relationship between Pakistan and Afghanistan. The Afghan government and U.S. officials have accused elements of Pakistan’s Inter-
Services Intelligence directorate (ISI) of helping to execute attacks in Afghanistan, including several recent high-profile strikes in Kabul.

The ISI has supported Lashkar-i-Jhangvi in the past, though it is not known whether the organizations maintain ties. Pakistan has denied any role in attacks in Afghanistan.

Relations between Pakistan and Afghanistan have been especially rocky in recent weeks, with Pakistan boycotting on Monday an international conference on the future of Afghanistan after a NATO airstrike last month killed 24 Pakistani troops.

Afghan President Hamid Karzai, who was in Germany to attend the conference, condemned the attacks and offered condolences to relatives of those killed.

“This is the act of enemies of Islam and Afghanistan who don’t want Afghan Muslims to live together and to be united,” he said in a statement. Karzai later canceled plans for a trip to Britain and flew back to Afghanistan.

Tuesday’s noontime attack in downtown Kabul was carried out by a suicide bomber who approached the Abul Fazal Abbas shrine in the Murad Khani district on foot as worshipers were streaming in, authorities and witnesses said. The blast killed at least 56 people, said Gen. Mohammad Zahir, director of criminal investigations for the Kabul police department. The American citizen killed in the attack was not a U.S. government employee, U.S. Embassy spokeswoman Megan Ellis told the Associated Press. The attack occurred during Ashura commemorations, the holiest occasion of the year for Shiites.

Shortly afterward, explosives attached to a bicycle killed four Shiite pilgrims in the northern city of Mazar-e Sharif. That bombing sparked clashes between Sunni and Shiite university students, witnesses and security officials said.

 Ahmad Shuja, a prominent Afghan analyst and blogger, said the attacks were likely to exacerbate ethnic tensions that have remained largely latent in the past decade, as fighting between the Western-backed Afghan government and the Taliban has eclipsed long-running ethnic conflicts. The majority of the victims in Tuesday’s attacks were ethnic Hazaras.

Human rights groups say the Taliban, a radical Sunni group, massacred thousands of Hazaras in the late 1990s when it held power in Kabul. Taliban leaders have derided Shiites as nonbelievers. 

 “The impact this has on the political process is it makes reconciliation much less of a choice,” Shuja said in a phone interview from Washington. “It increases antipathy within an aggrieved community and brings back memories of an era during which the Taliban carried out massacres.”

The attacks come at a crucial time for Afghanistan as its government pursues elusive peace talks with the Taliban. Facing fiscal woes and political fallout, the United States and allied nations with troops here intend to terminate the coalition’s combat mission in Afghanistan by the end of 2014.

Muktar Amiri, 28, a Hazara resident of Kabul, watched in disbelief as police officers cleaned up the site of the bombing Tuesday afternoon.

“This will have a big effect,” he said. “From now on, there will be some friction between Shiites and Sunnis.”

He said it was too early to say who was to blame but speculated that the motive was to lay the groundwork “for another civil war here when the Americans leave.”

The American commander of NATO troops in Afghanistan, without blaming the attacks on the Taliban, called on the group’s leader, Mohammad Omar, to denounce the bloodshed.

“This insurgency, which wraps itself in a false veil of Islam, must know that killing innocent pilgrims will spell its own demise,” Gen. John Allen said in a statement. “I call on Mullah Omar to condemn this grotesque act of terrorism.”

Zabiullah Mujahid, a Taliban spokesman, condemned the attacks, which he charged were probably orchestrated by the West to justify extending its presence in Afghanistan.

The suicide bomber arrived in Kabul with a group of Shiite pilgrims from Logar province, just south of the capital, according to Abdul Aziz, an Afghan intelligence official who was near the shrine when the blast occurred. He said authorities had no reason to search pilgrims as they walked into the shrine.

“When they come in a crowd practicing their religion, who can search them?” he said.

Shortly before the bomber struck, shirtless men were flagellating themselves and beating their chests, a gesture of penance practiced during Ashura, the climax of a month-long period of mourning commemorating the death of the prophet Muhammad’s grandson Hussein.

Abdul Ali Ahmat was inside the shrine distributing glasses of milk to worshipers when the blast occurred. He said the bomber struck when the mosque was particularly crowded, as worshipers trying to leave mixed with a large group that was entering.

“I saw women, children and men,” he said. “It was a horrible, horrible situation.”

Abdullah Khadri, a shopkeeper who was near the site of the attack, said it had undermined his confidence in the Afghan government.

“The police are too weak; the government is too weak,” he said. “They can’t even provide protection for our religious day.”

Correspondent Karin Brulliard in Islamabad, Pakistan, and special correspondents Javed Hamdard and Sayed Salahuddin in Kabul contributed to this report.