Pakistani rangers stand guard outside a hopsital where injured victims of target killing are brought to hospital in Karachi, Pakistan, May 13, 2015.[Photo/IC]

KARACHI, Pakistan - Gunmen on motorcycles opened fire on a bus in Pakistan's southern city of Karachi on Wednesday, killing at least 43 people, police said, in the latest attack directed against religious minorities this year.

Television channels carried pictures of a pink bus covered in bullet holes and lines of waiting ambulances.

"There were six attackers. They boarded the bus and carried out the shooting," Police Superintendent Najib Khan told Reuters.

He said all the passengers were from the Ismaili community, a minority Shi'ite Muslim sect in majority-Sunni Pakistan.

A splinter group of the Pakistani Taliban called Jundullah claimed responsibility.

"These killed people were Ismaili and we consider them kafir (non-Muslim). We had four attackers. In the coming days we will attack Ismailis, Shi'ites and Christians," spokesman Ahmed Marwat told Reuters.

At least 43 people had been killed and 13 wounded, provincial police chief Ghulam Haider Jamali told media.

Ayesha Aly, a member of the Ismaili community, wept on television as she explained that the bus picked up people for work and dropped them off every day.

"It was on its usual rounds when this attack happened. People were just getting on with their normal lives, out to earn a living," she said.

Taliban splinter groups have bombed several mosques 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 were killed in an attack on a Shi'ite mosque in the northeastern city of Peshawar, and 60 were killed in a January attack on a Shi'ite mosque in the southern province of Sindh.

Karachi, a megacity of 18 million that is Pakistan's financial heart, is also under the responsibility of the paramilitary Rangers, who report to the generals.