Afghan border police have killed seven civilians after opening fire on a group of villagers they mistook for fighters, according to a police official in Kandahar province.

The villagers were gathering wood near the Pakistan border before they were killed by gunfire on Friday, the official said.

The officers who shot the villagers were driving through Kandahar province's Shorabak district before sunrise on Friday when they spotted the seven men and thought they were about to be ambushed, General Abdul Raziq, commander of the border police of southern Afghanistan, said.

They started shooting from about 350 metres away and only discovered when they went to recover the bodies that no one was armed, he said.

All six officers involved in Friday's pre-dawn shooting have been arrested and the incident is under investigation, Raziq said.

The Afghan-Pakistan border area is a common transit route for both Taliban fighters and smugglers.

Border police are regularly attacked in the area.

Last year, 2,400 Afghan civilians were killed in attacks by Taliban fighters or in Afghan and NATO-led operations, according to the United Nations.