Afghan President Hamid Karzai says he plans to confront the Pakistani government over an unprecedented suicide bombing Tuesday that targeted Shi'ite Muslims in Kabul, killing 55 people.

Mr. Karzai said Wednesday that his government has launched an investigation, but that the attack originated in Pakistan with the Lashkar-e-Jhangvi militant group that is based there. The Sunni extremist group is affiliated with al-Qaida and is responsible for a number of attacks against Shi'ites in Pakistan.

So far, Islamabad has not responded to Mr. Karzai's statements.

President Karzai cancelled a trip to Britain to visit with the victims of the blast and their families.

Tuesday's blast in Kabul happened near the Abul Fazel shrine as Shi'ite 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, but it is not clear if Shi'ites were specifically targeted.

The Taliban denied involvement in Tuesday's attacks, calling them “cruel and indiscriminate” and blaming them on the “invading enemy.”

In a new attack Wednesday, officials say a roadside bomb hit a bus in southern Afghanistan's Helmand province, killing at least 19 civilians. Five children were among the dead.

Experts say the attacks, which came a day after the conference to discuss Afghanistan's future as international combat troops prepare to leave the country, underscores the importance of what was agreed in Bonn — that all countries in the region need to cooperate in securing long-term stability for Afghanistan.