Image copyright Afghan National Army Image caption Local officials say the man identified himself as German

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.

His identity is unclear. Officials say he speaks German and says he is 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.

Afghan officials believe the man has been with the Taliban for eight years. An army statement said: "The German national calls himself Abdul Wadood."

His German name is not known.

"The man has been moved to Kandahar air base and he is now in the custody of US forces," Maj Abdul Qadir Bahdurzai of the Afghan army's 215 Corps told BBC Afghan.

Photos of his capture show a man in his thirties or forties in traditional Afghan dress, flanked by two Afghan special forces soldiers.

"A man with a long beard, wearing a black turban who identified himself as a German citizen and speaks German was taken along with three other suspected Taliban on Monday night in Gereshk district of Helmand province," a spokesman for the provincial governor said.

Gereshk police chief Ismail Khplwak described the captured man as the "military adviser of Mullah Nasir", commander of a local Taliban elite group in Helmand, AFP news agency reported.

While foreign fighters are common in Syria alongside so-called Islamic State (IS) militants, it is rare to find Western nationals in Taliban ranks.

The best-known is perhaps John Walker Lindh, who became known as the "American Taliban" and was arrested in 2002 and sentenced to 20 years in prison.

Taliban insurgents control more territory in the country than at any time since foreign combat troops left in 2014, BBC research published in January showed.

It is estimated that about 15 million people - half the population - are living in areas that are either controlled by the Taliban or where the Taliban are openly present and regularly mount attacks.

Attacks in recent months claimed by Taliban and IS have killed scores in Kabul and elsewhere.

Recent major attacks in Afghanistan