By Mirwais Harooni

KABUL (Reuters) - Afghan President Hamid Karzai on Wednesday blamed a Pakistan-based group for bomb attacks in three Afghan cities that killed at least 59 people on Tuesday, an allegation that could stoke new tensions with Islamabad.

The blasts were the worst sectarian attacks in Afghanistan since the fall of the Taliban government. The largest explosion, at a shrine in the heart of Kabul, killed 55.

"Lashkar-e-Jhangvi which is based in Pakistan has claimed responsibility for this attack ... We will investigate the issue very carefully and will discuss it with the Pakistani government," Karzai said, after cancelling meetings in London to rush home and meet the victims.

"This is the first time in Afghanistan, on a very important religious day, they have acted. Without doubt it is hostile to Afghanistan, the people of Afghanistan, and Islam," Karzai said at a hospital treating victims of the Kabul blast.

Afghans have previously been spared the large-scale sectarian attacks that regularly trouble Iraq and neighboring Pakistan, but now face the grim prospect of a new type of bloodshed being added to the dangers of daily life.

A caller who identified himself as a spokesman for the Pakistan-based group Lashkar-e-Jhangvi al-Alami claimed responsibility for Tuesday's suicide attack in Kabul, in a telephone call to a Reuters reporter hours after the attack.

The caller, who identified himself as Abu Bakar Mansoor, said the Shi'ite community was the target.

However he was not known to the Reuters reporter, former militants once linked with the group said they did not recognize his name, and the claim could not be independently verified.

Lashkar-e-Jhangvi is one of Pakistan's most violent anti-Shi'ite groups and part of an al Qaeda-linked nexus of militants. It has also forged strong ties with militant groups operating in the tribal areas on the Afghan border.

The group emerged as a sectarian force in the 1990s, targeting minority Shi'ite Muslims, but graduated to more audacious attacks, like the truck bombing of Islamabad's Marriott Hotel in 2008 in which 55 people were killed.

In a separate incident on Wednesday, a roadside mine killed 19 civilians and injured another five when it exploded in the southern Afghan province of Helmand on Wednesday, the provincial government said, taking the death toll to 78 in just two days.

SECURITY FEARS

The NATO-led International Security Assistance Force (ISAF), responsible for security across much of the country, says it is turning the tide against the Taliban.

But if Tuesday's bombing sets a precedent for violence between the Sunni Muslim majority and the Shi'ite minority, it would severely stretch army and police resources.

At a funeral ceremony on Wednesday for victims of the attack, hundreds of Shi'ite Muslims bore aloft the bodies of the dead, chanting that because they had been killed at a Muslim ceremony, they had died in the name of the Prophet Mohammad.

"We were sacrificed for you," they shouted.

The interior ministry has blamed "the Taliban and terrorists," without giving further details, while the Taliban strongly condemned the bombings.

"We want the Afghan government, international community and those who are involved in Afghanistan's affairs to reveal those who were behind the attack," said Yazdan Parast, another Shi'ite Muslim attending the ceremony.

Among those killed in Tuesday's attacks was a U.S. citizen the American embassy in Kabul said in a statement. It gave no further details.

At the German conference, the Afghan government's Western backers, who have spent billions of dollars on the country since the U.S.-led overthrow of the Taliban government in 2001, pledged to support the country beyond the end-2014 deadline for the withdrawal of foreign combat troops.

Afghanistan has said that it will not be able to afford the army and police force it needs after 2014 without international help, and Tuesday's attack is likely to reinforce fears about the ability of Afghan forces to cope with violence after ISAF has fully handed over security.

(Additional reporting by Sayed Hassib and Emma Graham-Harrison; Writing by Daniel Magnowski; Editing by Sanjeev Miglani)