KABUL: Afghan President Hamid Karzai blamed Pakistani extremists on Wednesday for an unprecedented attack on Shia Muslims in Kabul that killed 55 people, demanding justice from the government in Islamabad.

By pointing the finger at the Lashkar-i-Jhangvi militant group, Karzai threatens to ratchet up tensions with neighbouring Pakistan just days after Islamabad boycotted the Bonn conference on the future of Afghanistan.

Afghan officials fear that Tuesday's killings in Kabul and the northern city of Mazar-i-Sharif could unleash Iraq and Pakistan-style sectarian violence ahead of the scheduled departure of Nato combat troops in 2014.

On Wednesday, another 19 people died in a roadside bombing in the southern province of Helmand, bringing to 78 the number of people killed in just over 24 hours. Four people were killed near a mosque in Mazar-i-Sharif.

“We will pursue this issue with Pakistan and its government very seriously,” said Karzai as he visited wounded survivors of the attack after cancelling a planned trip to Britain and rushing home from Germany.

There have been media reports of a purported claim from Lashkar-i-Jhangvi, which is linked to Al-Qaeda and blamed for killing thousands of Shias in Pakistan but not thought previously to have attacked within Afghanistan.

“Lashkar-i-Jhangvi is based in Pakistan, therefore the government of Afghanistan with all its strength and international support will pursue this issue, Afghanistan cannot ignore the blood of its children,” Karzai said.

Afghans traditionally blame Pakistanis for fuelling much of the violence in their country, where the Taliban are leading a 10-year insurgency against Karzai's government and 140,000 Western troops.

His spokesman Aimal Faizi told AFP that Karzai will “demand Pakistan take executive measures... so that justice can be done”.

There are reported links between Lashkar-i-Jhangvi, which was one of the groups involved in the kidnap and beheading of American journalist Daniel Pearl in Karachi in 2002, and Pakistani intelligence.

The US embassy in Kabul confirmed that an American was among those killed on Tuesday, in the Afghan capital where the victims were buried at peaceful mass gatherings with police on standby for violence which failed to materialise.

The 19 civilians killed in Wednesday's roadside bombing in Helmand included seven women from one family and five children, local officials said.

There was no immediate claim of responsibility for the blasts, which came as the civilians travelled from provincial capital Lashkar Gah to Sangin district, historically one of the most troubled in Afghanistan.

Lashkar Gah was brought under Afghan control in July and three other districts of Helmand are due to shift from Nato to Afghan hands within weeks as part of the phased withdrawal of most Western combat troops.

Tuesday's bombings were unprecedented in targeting a key religious holiday, Ashura, which is the holiest day in the calendar for Shias.

An Afghan official said the bomber who attacked the Kabul shrine was a Pakistani affiliated to Lashkar-i-Jhangvi, which is not previously known to have carried out any attacks in Afghanistan.

Officials and experts suggested that if Pakistanis did orchestrate the attacks, then elements in the Afghan Taliban may have helped facilitate them.

Sediq Sediqqi, a spokesman for the Afghan interior ministry, said the strikes were the work of “the Taliban and their associates”, adding that no-one else carried out such suicide attacks in Afghanistan.

An Afghan security official speaking on condition of anonymity said the bomber was a Pakistani connected to Sepah-i-Sahaba, a Lashkar-i-Jhangvi offshoot, who wanted to “inflame sectarian violence in Afghanistan”.

A Western security official also suggested Pakistani involvement but stressed it was not clear whether this was “institutional”, referring to reported links between militants and Pakistani intelligence.

“We're particularly looking at TTP (Tehreek-i-Taliban, the Pakistani Taliban) although at the moment we don't have any proof,” he said.

A Pakistani official speaking anonymously said Lashkar-i-Jhangvi was closely associated with the Pakistani Taliban.