Afghan President Hamid Karzai said Wednesday that the suicide bombing that targeted Shiite Muslim worshipers in Kabul and killed dozens of people was plotted in Pakistan.

Speaking at a hospital where victims of Tuesday’s attack outside a shrine in the Afghan capital were being treated, Karzai said he would demand answers from the Pakistani government about the bombing.

“We are investigating this issue, and we are going to talk to the Pakistani government about it,” Karzai told reporters. “Afghanistan cannot ignore the blood of all the victims in this incident.”

The accusation is likely to further strain the relationship between the neighboring countries.

A spokesman for Lashkar-i-Jhangvi, the Pakistani militant group that has carried out attacks against Shiites in Pakistan, asserted responsibility for the bombing, according to news outlets in Pakistan.

A senior Pakistani official said his government has no link with Lashkar-i-Jhangvi, the Associated Press reported.

“Lashkar-i-Jhangvi has declared war on the security forces in Pakistan,” Gen. Athar Abbas told the news service. “They are being hunted down.”

At least 56 people were killed in the bombing outside the Abul Fazal Abbas shrine, one of the deadliest attacks on civilians in the decade-long war in Afghanistan. An American citizen was among those killed, the U.S. Embassy said Wednesday in a statement. The embassy said its consular staff is providing support to the victim’s relatives, but it declined to identify the person.

Karzai said he believed Lashkar-i-Jhangvi’s claim, but he did not offer details. The group is linked to the Taliban and al-Qaeda.

The attack in Kabul and a second one Tuesday in the northern city of Mazar-e Sharif that killed four people marred the commemoration of Ashura, the holiest day of the year in Shiite Islam.

On Wednesday, Afghan officials said at least 19 civilians were killed in Helmand province, in the south, after their van detonated a land mine. The blast happened about 10 a.m. in the Sangin district, according to the governor’s office.

Land mines intended for Afghan and foreign troops have killed hundreds of civilians.

“Placing bombs on the roads used for public commuting demonstrates the enemy’s cruel nature to deliberately target people,” the Afghan government said in a statement.