A day after a suicide bomber targeting Shi'ite Muslims killed 55 people in the Afghan capital, Kabul, Afghan President Hamid Karzai vowed to confront neighboring Pakistan about the unprecedented attack.

Mr. Karzai, who cancelled a planned trip to the United Kingdom and returned home in the wake of the killings, said Wednesday he blames the Pakistan-based Lashkar-e-Jhangvi group for the bombing. The Sunni extremist group is affiliated with al-Qaida and is responsible for a number of attacks against Shi'ites in Pakistan.

Pakistani authorities have been hunting members of the outlawed group for years. Pakistani Interior Minister Rehman Malik on Wednesday pledged to take action if Afghan authorities offer any proof to support their claim. But he also warned against making “baseless accusations.”

Tuesday's blast in Kabul took place near the Abul Fazel shrine as worshippers gathered on Ashura, the holiest day of the Shi'ite Muslim calendar. A second explosion killed four people at a shrine in the northern city of Mazar-e-Sharif.

The Taliban condemned the killings, calling them “cruel and indiscriminate” and blaming them on the “invading enemy.” The group also denied any involvement in the attacks, but Lashkar-e-Jhangvi appears to have claimed responsibility for the Kabul carnage.

On Wednesday, a roadside bomb in southern Helmand province hit a bus, killing at least 19 more Afghan civilians. Five children were among the dead.

Experts say the attacks, which came in the aftermath of an international conference to discuss Afghanistan's future following the withdrawal of coalition combat troops by the end of 2014, underscore the importance of what was agreed in Bonn — that all countries in the region need to cooperate in securing long-term stability for Afghanistan.

In New York Wednesday, the United Nations Security Council issued a statement, condemning “the abhorrent terrorist attacks” and expressed deep sympathy for the victims and their families.

The statement said that “terrorism in all its forms and manifestations constitutes one of the most serious threats to international peace and security.” The council members reiterated their determination to combat all forms of terrorism, as well as their support of the people and the government of Afghanistan.