An Afghan official says a police officer in western Farah province opened fire at fellow policemen, killing four before he was taken into custody.

Mohibullah Mohib, a spokesman for the provincial police chief, says an investigation is underway into the attack, which occurred on Monday night.

Mohib says it happened at a checkpoint near the city of Farah, the provincial capital.

No group immediately claimed responsibility but the Taliban have been very active in Farah. Mohib says the investigation will show whether there are links to the Taliban.

The attack comes as a resurgent Taliban, who now hold sway over nearly half of Afghanistan, stage near-daily assaults on Afghan military and security forces, government and other installations throughout the country, including in Farah.