KARACHI (AFP) - The Islamic State in Iraq and Syria (ISIS) group on Wednesday claimed responsibility for an attack by gunmen on a bus that killed at least 43 Shiites in Pakistan's biggest city Karachi.

"Thanks be to Allah, 43 apostates were killed and around 30 were wounded in an attack carried out by Islamic State soldiers on a bus transporting Shiite Ismaili infidels in the city of Karachi," the extremist group said in a statement posted on Twitter accounts, using an ISIS alternative name.

It was the first official claim of responsibility by the ISIS leadership of an attack in the Afghanistan-Pakistan region.

The Jundullah militant faction, a splinter of the Pakistani Taleban, said earlier it was responsible for the massacre while police said they also found leaflets at the scene claiming the attack on behalf of the ISIS.

Pakistan has experienced a rising tide of sectarian violence in recent years, particularly against Shiites, who make up around 20 per cent 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 in the eastern district of Malir.

"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.

Prince Karim Aga Khan, the spiritual leader of the world Ismaili community, confirmed the toll in a statement sent by his office in France.

"This attack represents a senseless act of violence against a peaceful community," the philanthropist and business magnate said.

The killings were also condemned by Prime Minister Nawaz Sharif and army chief Raheel Sharif, while the provincial government announced a day of mourning Thursday.

A spokesman for the Jundullah militant faction, which has claimed several major attacks in the past including one on a church in Peshawar that killed 81 Christians in 2013, said his group was responsible for storming the bus.

Speaking over the phone from an undisclosed location, Ahmed Marwat said four, not six fighters, had participated, adding: "Shiites and Ahmadis are unbelievers, they are apostates and deserved death." Little is known about Jundullah, its strength, or its organisational structure, but it is believed to be based out of the country's tribal areas and parts of Karachi.

A security official at the scene also 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.

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.

Wednesday's attack was the second deadliest in Pakistan this year after 62 Shiite Muslims were killed in a suicide bombing in late January.

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.

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 Taleban 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 fallen significantly 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.