Unidentified gunmen have killed at least nine people in the south-western Pakistani city of Quetta, police say.

The gunmen ambushed a military vehicle near a railway station. Police say most of the dead are soldiers. Four people have been wounded in the attack.

The shooting came hours after a visit to Quetta by the US Assistant Secretary of State Richard Boucher.

Officials have blamed the attack on tribal rebels who have waged a low-key insurgency in the area.

"The troops were returning from home leave when some unknown gunmen ambushed their van near a railway station and according to my information, nine people were killed," Maj Gen Waheed Arshad told Reuters news agency.

Demanding autonomy

Quetta police chief Rehmatullah Niazi said the gunmen attacked the van carrying soldiers at about midnight and fled the scene.

It is not yet clear who was behind the attack, but according to Reuters, a man claiming to speak for the Balochistan Liberation Army, a guerrilla group, called a press club in Quetta to claim responsibility for the attack.

Quetta is the capital of Balochistan province, which has witnessed frequent attacks on security forces and gas pipelines by tribal rebels demanding greater autonomy and a larger share of the profits from natural resources.

US Assistant Secretary of State Richard Boucher visited Balochistan hours before the attack.

He met government leaders in Quetta as well as opposition and election officials.

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Quetta bodies