At least 43 people have been killed and 20 injured in a gun attack on a bus carrying Ismaili Shia Muslims in the Pakistani city of Karachi, police say, the BBC reports.

A police official said six gunmen on motorcycles stopped the bus and fired indiscriminately at passengers.

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.

About 60 people were on board the bus when it was stopped in the Safoora Goth area of Karachi on its way to an Ismaili Shia place of worship, police said.