Kabul, Afghanistan (CNN) -- Afghan President Hamid Karzai put pressure on Pakistan on Wednesday to help investigate an unprecedented attack on Shiite Muslim worshippers in Kabul a day earlier, saying his country would pursue the probe "with all the power we have."
A militant group based in Pakistan claimed responsibility for the bombing, which killed at least 56 people on Tuesday, the Shiite holy day of Ashura.
"To kill children on Ashura is to be an enemy to humanity. To kill Muslims on Ashura is to be an enemy to Islam," Karzai said, calling the massacre "unique in Afghan history."
"Lashkar-e-Jangvi has announced they are our enemies and of Afghans and Islam and humanity," Karzai said, while visiting a hospital to meet survivors of the attack.
"We will follow it very seriously and will definitely pursue it with the government of Pakistan, because it relates to our lives and the lives of these children lying in the hospital," the youngest of whom was a boy one and a half years old, he said.
Karzai has not confirmed the group carried out the attack, spokesman Aimal Faizi emphasized, but based his remarks on its claim that it did.
The mass-scale sectarian attack on Shiite worshipers was unlike anything the country has seen in its decade-long war -- in contrast to Iraq, where violence between Shiites and Sunnis has been a major feature of the conflict.
At least 193 people were wounded in addition to those killed when a suicide bomber detonated a device at a Shiite shrine in Kabul on Tuesday, Afghan Health Ministry spokesman Kargar Norughli said.
Four people also were killed in a Tuesday explosion at a roundabout on a busy street in Mazar-e Sharif, the provincial capital of Afghanistan's northern Balkh province, police official Lal Mohammad Ahmadzai said. Another 21 were wounded in that attack.
"The enemies tried to spread fear in this important holiday in the city," Ahmadzai said.
It was not immediately clear whether the attack in Mazar-e Sharif was linked to the attack in Kabul.
The U.N Security Council condemned both attacks "in the strongest terms" in a statement Wednesday and urged Afghanistan to hold those responsible to justice, as well as expressing its condolences to the victims.
Karzai also strongly condemned an attack in Helmand province on Wednesday, he said in a statement.
At least 19 people, including women and children, were killed when their bus hit a roadside mine Wednesday, said Daud Ahmadi, a spokesman for the governor of Helmand province.
Their bus was traveling from provincial capital Lashkar Gah to Sangin district in the south of the restive province, he said.
He said he did not know whether the civilians were the target of the roadside mine planted by the Taliban.
Karzai canceled a visit to the United Kingdom after the Tuesday blasts.
A spokesman for the Afghan Embassy in London said Karzai had been due in London late Tuesday from Germany but was instead flying back to Afghanistan.
Megan Ellis, a U.S. Embassy spokeswoman in Kabul, said Wednesday that an American was among the dead. She added that consular officials were in touch with the family, who would need to consent to the release of further details.
The Taliban denied involvement in Tuesday's attacks.
A man identifying himself as a spokesman for Lashkar-e-Jangvi al Almi, a group with links to al Qaeda and the Pakistan Taliban, made the claim of responsibility in a call to Radio Mashaal, a Pashto-language station in Pakistan sponsored by the U.S. government.
The group is an offshoot of the powerful Lashkar-e-Jangvi, which has a record of high-profile suicide bombings in Pakistan, including the attack on the Marriott Hotel in Islamabad in 2008.
Afghanistan has seen previous attacks on mosques. In 2006, rioting broke out between Shiites and Sunnis at an Ashura festival in Herat, leading to several deaths. But the country has not seen sectarian attacks of the scale that occurred Tuesday.
Ashura commemorates the martyrdom of Hussein, grandson of the Prophet Mohammed. Hussein's death in battle in Karbala, Iraq, in 680 is one of the events that helped create the schism between Sunnis and Shiites, the two main Muslim religious movements. Shiites are a minority presence in Afghanistan, which is predominantly Sunni.
CNN's Tim Lister, Mohammed Tawfeeq, Mick Krever and Josh Levs contributed to this report.