KABUL: Six Afghan policemen were found shot at their checkpoint in the country's south and two other policemen were missing, raising suspicions they killed their comrades, an official said Thursday.

District chief Nayamatullah Samim said the policemen's bodies were discovered in the shelter near their checkpoint Wednesday night, after they did not check in with their superiors.

Two other officers assigned to the checkpoint were missing, along with vehicles and weapons.

''We don't know yet whether these two missing men are the ones who killed the police or if they were taken by other attacks,'' Samim said.

So-called ''insider attacks'' in which Afghan forces open fire on their comrades or international forces threaten to shake the confidence of the two sides as the 2014 withdrawal of most of the international troops approaches.

Taliban insurgents have warned they would infiltrate Afghan security forces to carry out insider attacks. There have been several such attacks in the past year, including officers poisoned while eating.

The most recent such attack was May 28, when two recently rehired Afghan police opened fire on their commander at a checkpoint in a remote district in the country's south, killing him and six of his men before fleeing in a police vehicle with weapons.

On May 5, an Afghan soldier turned his weapon on two US Marines he was working with and killed them.

The insurgents have stepped up attacks across the board in recent weeks, bringing violence to among the highest levels of the 12-year war as they seek to weaken the government and security forces.

Foreign troops have been pulling back from front-line fighting in preparation for the international coalition turning over full responsibility for security to Afghan units later this year.