Afghan security forces say they have arrested a German man in southern Helmand province who has been a long-time military adviser to the Taliban.

The man was captured with other three other suspected militants in a raid on a bomb-making site in Gereshk district. Afghan officials say, his identity is unclear and speaks German.

If confirmed, it would be a rare case of a westerner fighting with insurgents in Afghanistan. Large parts of Helmand are under Taliban control.