At least 43 people have been shot dead by men on motorbikes who attacked a bus in Karachi, according to police in Pakistan.

Television channels showed pictures of a pink bus covered in bullet holes.

Police are said to be hunting at least six attackers.

Provincial police chief Ghulam Haider Jamali said 43 people had been killed, including 16 women.

He said the bus was on its way to a place of worship for Ismaili Shia Muslims when gunmen boarded it and sprayed the passengers with bullets.

"These are the people who are extremists, who are terrorists," he said. "These are the same people who have been doing terrorism before."

No one has so far claimed responsibility.

The Taliban and other Sunni Muslim extremist groups, which view Shiites as apostates, have long had a presence in Karachi and have targeted Shiites in the past. Ismaili Shiites are a tiny minority in mostly Sunni Pakistan.

Sindh province Chief Minister Qaim Ali Shah said: "They were innocent people. We feel very sorry for this ghastly act."

A separate senior police official told AFP: "The gunmen stopped the bus and first fired at it from outside. Then they entered inside the bus and open fire indiscriminately. After that they checked to see if anyone was left uninjured.

"The bus had a capacity of 52 passengers but it was overloaded and dozens of people were boarding it. Most of them were from (the) Ismaili community."

An official of the Ismaili National Council, a group which represents the community, said the bus was owned by the community's welfare board.

Pakistan has seen a rise in sectarian violence in recent years, particularly against minority Shias who represent around 20% of the country's predominantly Muslim population of 200 million.