At least 43 people were killed in a bus attack on Wednesday targeting members of Pakistan's tiny Shia Muslim Ismaili community in Karachi.
A senior police officer in the southern port city said gunmen arrived on motorcycles and boarded the bus before firing on passengers. "The passengers were targeted from close range. They were totally defenceless and at the mercy of the killers," the officer said.
Jundollah, a splinter group of the Pakistani Taliban, claimed responsibility, according to Reuters.
"These killed people were Ismaili and we consider them as Kafir [non-believer]," the news agency quoted Ahmed Marwat, a Jundollah spokesman, as saying. "We had four attackers. In the coming days we will attack Ismailis, Shi'ites and Christians."
Analysts said the attack reflected the extent to which Pakistan's Taliban militants have spread beyond the rugged regions along the Afghan border.
"As the Pakistan army steps up its campaign against the Taliban, the Taliban will look for ways to fight back," said retired Major General Mahmood Durrani, a former national security adviser to the prime minister's office.
Taliban splinter groups have bombed several sites belonging to religious minorities this year.
In March, suicide bombings outside two churches in Lahore killed 14 people and wounded nearly 80. Days later, a bomb after Friday prayers wounded 12 people outside a minority Bohra mosque in Karachi.
In February 20 people died in an attack on a Shia mosque in Peshawar, the northeastern city where at least 141 pupils lost their lives in a massacre at a military-run school in December. In January, 60 people were killed in an assault on a Shia mosque in southern Sindh province.
In recent months Pakistan's Shia have voiced growing concern over the fallout of an increasingly intense clash between mainly Sunni Saudi Arabia and Houthi rebels in Yemen backed by predominantly Shia Iran.
Many Shia community leaders have opposed suggestions of Pakistan committing troops to the Saudi-led campaign, arguing that such a deployment would prompt further attacks on Shia groups by local hardline Sunni militant groups.
"There is a very real threat if we get involved in a Middle East conflict," said Ghazi Salahuddin, a newspaper commentator. "We have to remain neutral because compromising our neutrality will have consequences for Pakistan's internal stability."
Western diplomats said Wednesday's attack was a powerful reminder of the acute security challenges faced by Pakistan, despite recent indications of an economic recovery.
"Unless there is convincing evidence of security conditions improving rapidly, Pakistan's economic improvements will always be surrounded by suspicion," said one.
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