KARACHI (AFP) - Gunmen on Saturday hurled a firecracker and opened fire at a primary school in the southern Pakistani city of Karachi, killing its principal and wounding eight children, police said.

The children were aged between five and ten while the headmaster was a member of the Awami National Party, which consists mainly of ethnic Pashtun migrants from the country's north-west, police added.

"The principal of the private school was killed and eight children were wounded when unknown gunmen fired at school gate and threw a cracker inside the school," senior police official Imran Shaukat told AFP.

The gunmen fled on motorbikes and nobody has claimed responsibility for the attack, he said, adding that police have started an investigation.

The attack took place in the low-income Ittehad Town neighbourhood of Karachi - a port city of 18 million people plagued by murders, kidnappings and politically linked violence.

Pakistan is due to hold elections in May but a surge in violent sectarian attacks against minority Shiite Muslims, including a bombing in Karachi earlier this month which killed 50 people, has raised concerns over security at the polls.