Gunmen on motorcycles killed 41 people in an attack on a bus in Pakistan's southern city of Karachi on Wednesday, police said, in what appeared to be the latest violence directed against religious minorities this year.

"There are 41 deaths, men and women included," police spokesman Atiq Shaikh told Reuters.

Television channels carried pictures of a pink bus covered in bullet holes and lines of waiting ambulances. Police said the passengers were from a religious minority.

No one immediately claimed responsibility for the attack.

Taliban splinter groups have bombed several mosques belonging to religious minorities this year. (Reuters)