At least 43 members of Pakistan's Shiite Ismaili minority were killed when gunmen opened fire on their bus in Karachi on Wednesday, police said, with a leaflet at the scene claiming the attack on behalf of the Islamic State group.
It was the second deadliest militant attack in Pakistan this year after 62 Shiite Muslims were killed in a suicide bombing in late January.
Pakistan has seen a rising tide of sectarian violence in recent years, particularly against Shiites, who make up around 20 percent of the country's predominantly Muslim population of 200 million.
"According to the initial information which we have received from hospitals, 43 people have been killed and 13 wounded," Ghulam Haider Jamali, police chief of Sindh province told reporters at the site.
"Six terrorists came on three motorcycles, they entered the bus and began firing indiscriminately. They used 9mm pistols and all those killed and injured were hit by the 9mm pistols," he said.
A senior member of the Ismaili National Council, a community group that represents the Ismaili branch of Shiites confirmed the toll.
A security official later showed AFP a copy of a torn and blood-stained pamphlet claiming responsibility on behalf of the Islamic State group -- the second time in as many months that such material has been discovered at the scene of an attack in Karachi.
Police handed over similar leaflets to reporters after the shooting of US national Debra Lobo, a member of faculty at the city's Jinnah Medical and Dental College, on April 16, but analysts remain doubtful over their authenticity.
The leaflets, seen by AFP, are plain printed text documents with no IS emblems or insignia and there has been no confirmation from the group's leadership in the Middle East that it has carried out any attacks inside Pakistan.
- Anguished relatives -
At the city's Memon Hospital Institute, where most of the wounded were rushed, crying relatives formed a human chain outside the main building to keep onlookers away.
A sobbing middle-aged man told AFP: "I have come to collect the body of my young son. He was a student preparing for his first year exams at college."
The bus itself, which had been driven after the attack to the hospital, was blood-drenched and riddled with bullet holes.
Anti-Shiite attacks have been increasing in recent years in Karachi and also in the southwestern city of Quetta, the northwestern area of Parachinar and the far northeastern town of Gilgit.
Around 1,000 Shiites have been killed in the past two years in Pakistan, with many of the attacks claimed by the hardline Sunni group Lashkar-e-Jhangvi (LeJ) who view them as heretics.
Ismaili Shiites are known for their progressive Islamic views. Their spiritual leader Prince Karim Aga Khan is a globally renowned philanthropist and business magnate.
- IS concerns -
The attack comes as Pakistan steps up its efforts against militants following a Taliban massacre of 150 people, mainly children, in a school in Peshawar last year.
In the aftermath of the attack the government ended a six-year moratorium on executions, passed legislation to create military courts for terrorism cases, and pledged to crack down on all militant groups.
There have also been concerns about the IS group tapping support in Pakistan, a country awash with dozens of militant groups.
Leaflets calling for support of IS jihadists have been seen over recent months in parts of northwest Pakistan and pro-IS slogans have appeared on walls in several cities.
Some disaffected Pakistani Taliban cadres have also said they have switched allegiance to IS, but the true extent of links to the group's Middle East operations remains unclear.
Karachi, a sprawling city of roughly 20 million, has long had a reputation for high crime rates as well as ethnic, political and sectarian violence.
But the violence has significantly fallen since 2013 after police and paramilitary rangers launched a crackdown that rights activists say has led to extra-judicial killings of suspected criminals and militants.