Suspected militants have murdered 18 Shiite Muslim men in a usually quiet region of Pakistan.

The men were hauled off busses by gunmen disguised in military clothing and shot in cold blood on Tuesday, authorities said.

The attack, which has been blamed on Islamist militant groups, occurred in the northern district of Kohistan, which neighbors a former Taliban stronghold in the Swat valley.

According to police, the attackers flagged down busses and clambered on board, asking passengers if they were Shiite or Sunni before dragging them outside and shooting them, Agence France Presse reported.

The motive was sectarian. The gunmen were wearing army uniform, Mohammad Ilyas, the police chief in Kohistan told AFP after the attack near the town of Harban.

They checked the identity of the passengers, got the Shiites off the vehicles and shot them dead.

The dead were all male, he added.

The attack comes after international groups criticized the Pakistani government for not tackling the sectarian violence between the country's majority Sunni and minority Shiite communities, which has claimed thousands of lives.

Tuesday's attack was the fourth in Pakistan's northwestern Khyber Pakhtunkhwa province since Thursday, raising the specter of a resurgent Taliban insurgency in the region.

In 2009 Pakistan managed to clear the Swat Valley, putting down a two year Taliban insurgency.