Karachi, Pakistan (BBN)-At least 45 people have been killed and 13 injured in a gun attack on a bus carrying Ismaili Shia Muslims in the Pakistani city of Karachi, police say.
A police official said six gunmen on motorcycles had stopped the bus and fired indiscriminately at passengers, reports BBC.
Prime Minister Muhammad Nawaz Sharif has condemned the attack and ordered an investigation.
A splinter group of the Pakistani Taliban called Jundullah has said it carried out the attack.
'DRESSED AS POLICE'
A leaflet was also left at the scene claiming the so-called Islamic State group was responsible.
It is the second deadliest militant attack in Pakistan this year after 62 Shia Muslims were killed in a suicide bombing in January.
This is the first such attack on the Ismaili community in Pakistan, and it is shocking even by the standards of Karachi.
The sheer numbers of casualties and the manner in which they were killed provide an uneasy contrast with the peaceful image of this Shia sect, which makes up a tiny proportion of Pakistan's mostly Muslim population.
One of the country's many militant groups has said it carried out the attack.
But we are not likely to find out for sure who did it because Karachi is in the throes of an armed struggle between the military and a confusing array of political, religious and sectarian militants.
Ismailis consider the Aga Khan their hereditary spiritual guide.
They mostly inhabit the Himalayan region of northern Pakistan, but also have a significant presence in Karachi where they run businesses and charities, and tend to use community-built accommodation and transport.
The attack is likely to cause some diplomatic tensions for Islamabad as the Aga Khan has been a major source of development funds.
This may be one reason why both the prime minister and the army chief have set aside other engagements to head for Karachi.
About 60 people were on board the bus when it was stopped in the Safoora Chowk area on its way to the Ayesha Manzil Ismaili centre in the north of the city, police said.
Relatives of some survivors said the gunmen had been dressed as policemen and they fled after the assault.
They said the bus driver was killed and an injured passenger drove the bus to Memon Hospital Institute. Witnesses said it was riddled with bullets and the interior was covered in blood.
BBC Urdu service editor Aamer Ahmed Khan in Karachi said it seemed to be a well-planned attack.
He said the bus was making one of five daily scheduled trips between a gated community housing mainly Ismailis on the north-eastern outskirts and the main city.
One man at the hospital told AFP news agency: "I have come to collect the body of my young son. He was a student preparing for his first year exams at college."
Ismaili spiritual leader Prince Karim Aga Khan said in a statement the "attack represented a senseless act of violence against a peaceful community".
Pakistani Prime Minister Nawaz Sharif described the killings as "a deplorable attempt to spread chaos in Pakistan".
A Jundullah spokesman said it had carried out the attack because it considered the victims to be infidels. He threatened more attacks in the coming days against Ismailis, Shias and Christians.
Pakistan's Army chief Gen Raheel Sharif has cancelled a three-day trip to Sri Lanka and is leading the investigation into the attack, the BBC's Urdu editor said.
Suicide bombings outside two churches in Lahore in March killed 14 people and wounded nearly 80; days later, a bomb after Friday prayers wounded 12 outside a minority Bohra mosque in Karachi.
Karachi, Pakistan's commercial capital of about 20 million people, has long had a reputation for sectarian violence.
WHO ARE THE ISMAILIS?
Ismaili Shias, in common with other Shia Muslims, revere Ali, son-in-law of the Prophet Mohammed (PBUH), but they also revere the Imam Ismail who died in 765 AD
They interpret the Koran symbolically and allegorically
They live in more than 25 different countries
Spiritual leader Prince Karim Aga Khan is a philanthropist and business magnate. He gives his name to bodies including a university, a foundation, and the Aga Khan Program for Islamic Architecture at Harvard University and the Massachusetts Institute of Technology
He has encouraged Ismailis settled in the industrialised world to contribute to those communities
Pakistan is about 20% Shia and 70% Sunni.
RECENT ATTACKS ON SHIAS IN PAKISTAN INCLUDE:
In January, an attack on a Shia mosque in the southern province of Sindh killed 60 people
In February, 20 people were killed in an attack on a Shia mosque in Peshawar