Kabul, Afghanistan ? The fallout from this week’s deadly suicide bombing in Kabul has further splintered Afghanistan’s relations with neighboring Pakistan and set back the U.S.-led military campaign to stabilize the region before international troops leave at the end of 2014.

The attack that killed 56 people and wounded more than 160 others outside a Shiite shrine highlighted a marked decline in security in the Afghan capital over the past year. Afghan forces, who have been in charge of security in Kabul for more than a year, have had successes in foiling plots and minimizing casualties, but insurgents increasingly slip through checkpoints and conduct complex assaults.

Ties between Afghanistan and Pakistan were already frayed when President Hamid Karzai on Wednesday blamed a Pakistan-based extremist group for the bombing at the shrine. Pakistan challenged Karzai to provide hard evidence.

The evidence, Karzai suggested, was that a man claiming to be from Lashkar-e-Jhangvi al-Alami, a Pakistan-based splinter group of Lashkar-e-Jhangvi that has carried out attacks against Shiite Muslims in Pakistan, called various media outlets Tuesday to claim responsibility for the Kabul bombing and a nearly simultaneous attack that killed four Shiites in the northern Afghan city of Mazar-e-Sharif.

“We are investigating this issue and we are going to talk to the Pakistani government about it,” Karzai said, standing outside a Kabul hospital after visiting victims of the bombing.

Until now, the decade-long Afghan war has largely been spared sectarian violence, where civilians are targeted simply for their membership in a particular religious group. Tuesday’s attack suggests that at least some militant groups may have shifted tactics, taking aim at ethnic minorities such as the Hazara who are largely Shiite and support the Afghan government and its Western partners.

But there was some doubt that a little-known splinter group could carry out the coordinated bombings in Afghanistan, where neither it nor the main Lashkar-e-Jhangvi has a history of conducting operations.

Pakistani Foreign Ministry spokesman Abdul Basit said Pakistan would encourage Kabul to share any evidence it has that the group was responsible.

Later, he sent a text message to The Associated Press, condemning the attack on the shrine.

“The government and the people of Pakistan are grieved and stand by the brotherly people of Afghanistan at this difficult time,” he said.

Pakistani military spokesman Gen. Athar Abbas dismissed any suggestions that Lashkar-e-Jhangvi has links to the country’s intelligence agencies or that the government was not doing everything it could to quash the group.

“Lashkar-e-Janghvi has declared war on the security forces in Pakistan,” he told the AP. He said the group has been implicated in some of the worst attacks on Pakistani security forces. “They are being hunted down,” he said.

Karzai began to sharpen his criticism of Pakistan several months ago after a suicide bomber, pretending to be a peace emissary from the Taliban, assassinated former President Burhanuddin Rabbani, who was leading Afghan efforts to broker a deal with the insurgency.

Afghan officials said the Sept. 20 assassination was planned on the southern outskirts of Quetta, the Pakistani city where key Taliban leaders are based. Afghan officials shared evidence with Pakistani officials, but Afghan-Pakistan cooperation on the investigation into Rabbani’s murder so far has been tenuous.

American-Pakistan ties are deeply scarred too. To protest recent NATO airstrikes that killed 24 Pakistani troops along the Afghan border, Islamabad boycotted this week’s conference in Bonn, Germany, on the future of Afghanistan. Pakistan also closed down routes that NATO members use to deliver supplies to the 130,000-strong coalition force in landlocked Afghanistan.

The recent events have left the three main players in the decade-old conflict — Afghanistan, Pakistan and the U.S. — at odds at a time when the West is trying to secure Afghanistan’s future — and convince its people that their security forces can keep the peace when most foreign troops go home in three years.

Previous attacks in Kabul have been blamed on the Haqqani militant network, which is based in Afghanistan and is thought to have ties to the Taliban, al-Qaida and Pakistan’s spy agency.

To the mourners burying the dead in Kabul and planting red flags in dusty cemeteries to mark the fresh graves, achieving stability in Afghanistan is more important than which militant group was responsible for the latest attacks.

Mohaqeq Zada, a member of the Shiite council in Kabul, said the bombing showed no one can count on the government for protection.

“There have been so many attacks, even against government officials, and still they can’t stop these things,” Zada said.

In London, Britain’s Defense Secretary Philip Hammond told legislators that it was too early to say if Tuesday’s attack — the first major sectarian assault in Afghanistan since the fall of the Taliban regime a decade ago — would derail plans to hand over security responsibilities to Afghan forces in an increasing number of regions.

But he acknowledged that recent incidents have raised concern.

“Clearly, the last two weeks have seen some significant setbacks,” Hammond said. “Yesterday’s attack in particular was a very unwelcome development — effectively, on the face of it, opening a new front” to the war, Hammond said. “But I think it would be hugely premature to suggest that that will deflect the process of transition to Afghan-led security.”

Associated Press writers Rahim Faiez in Kabul, Kathy Gannon in Islamabad, Pakistan, and David Stringer in London contributed to this report.