Gunmen opened fire on a bus carrying Shiite Muslim employees of Pakistan's space agency in this southern port city on Friday, killing six and wounding at least six others, police said.

The attack occurred as about 20 workers were on their way to a mosque for Friday prayers, said Athar Rashid Butt, a senior police official. The gunmen were on motorcycles and fled after the shooting.

Butt said six people were killed. Two wounded people were listed at a hospital in critical condition, he said.

The bus belonged to the Space and Upper Atmosphere Research Commission, or SUPARCO (search), which is charged with developing satellite technology, police said. The Shiites apparently worked for the agency.

Nobody claimed responsibility, but suspicion fell immediately on one of several Sunni Muslim extremist groups that have killed hundreds of minority Shiite Muslims (search) in recent years.

The attack occurred in a neighborhood in the west of the city, near the city's port.

Police said an investigation was under way, but no arrests had been made.

Hasan Turabi, a prominent Shiite leader, condemned the attack and asked the government to provide better protection for Shiite Muslims.

"It is the duty of the government to protect us from these terrorists," he said.

Most of Pakistan's Shiite and Sunni Muslims live peacefully together, but extremist groups on both sides are responsible for many attacks.

Sunni Muslim suicide attackers stormed a Shiite mosque in the southwestern city of Quetta (search) in July, killing some 50 worshippers.

A month earlier, gunmen opened fire on Shiite police trainees in the town, killing 11.