Gunmen in Pakistan's restive Balochistan province have attacked two buses, killing at least 19 passengers, officials have said.

The province's Home Minister Sarfraz Bugti said unidentified militants boarded the vehicles travelling to Karachi on Friday evening.

Passengers were then ordered off and shot dead, officials said.

Balochistan has seen a long-running conflict between separatists and security forces.

No group has yet said it carried out the attack on the two buses, which happened near the town of Mastung, south of the provincial capital, Quetta.

"The armed men were wearing the uniforms of the security forces,"  Bugti told Reuters.

Pashtun victims

An operation was launched after reports of the incident emerged, and at least five people have been rescued, officials say.

However, the number of passengers abducted from on board remains unclear,Dawn newspaper reports.

Fighting is ongoing in the area between the attackers and security forces.

Those killed were mostly ethnic Pashtuns, a local official told the BBC. They are natives of northern Balochistan.

While separatists have regularly attacked government and military targets, attacks on Pashtuns are a rarity, the BBC's M Ilyas Khan in Islamabad says.

He says officials believe this may be a tactical move by insurgents to put pressure on security forces who have recently carried out several "search and kill" operations against insurgents in the region.

Rebels in Balochistan have been fighting for greater autonomy from Islamabad, and a bigger share of the region's natural resources.