Karachi, Pakistan ? Attackers stormed into a Shiite mosque in southern Pakistan and sprayed worshippers with automatic weapons fire Saturday, killing at least nine people — including a 7-year-old boy — and wounding nine others, police and hospital officials said.

The gunmen jumped off motorcycles and rushed into the Imam Bargha Mehdi mosque shortly after the call to evening prayers in this southern port city and fired upon worshippers, witnesses said.

“The call for prayer had just begun and four people on two motorcycles rode up to the gate and opened fire,” said worshipper Mohammed Ali.

The motive for the killings was not immediately clear, but Pakistan has been wracked by religious violence in recent years, mostly by Sunni Muslim extremist groups targeting minority Shiites. Places of worship often are targeted.

The attackers had been waiting at a nearby tea shop and quickly fled the scene after the firing spree, witnesses said. No one immediately claimed responsibility for the attack.

“The government will leave no stone unturned to arrest these killers,” said regional Home Department minister Syed Sardar Ahmed. He said the government would increase security at mosques in the city.

Karachi, a sprawling industrial city, also has been the site of a series of violent attacks against Westerners and minority Christians in recent months.

A June 14 suicide bombing outside the U.S. Consulate in Karachi killed 12 Pakistanis and injured 50 others, while a May 8 suicide bombing outside the Sheraton Hotel in Karachi killed 11 French engineers and two other people.