A young couple who tried to elope in one of the most lawless and conservative parts of Afghanistan have been publicly executed by Taliban gunmen after their parents handed them over to be tried by insurgents.

Officials from the south-western province of Nimroz say Gul Pecha, in her late teens, and her boyfriend Abdul Aziz, 21, were shot by a firing squad outside a mosque in their home village of Lokhi on Monday.

The couple had fled to a nearby village and were planning to start a life together without the permission of their parents, according to the province's police chief Abdul Jabar Pardeli.

But they were found by their parents and turned over to the Taliban, who held them for four days in Lokhi's mosque before putting them on trial.

Ghulam Dastageer Azad, the governor of Nimroz, said the couple's execution was "against Islam, against the law and against the constitution".

An unofficial justice system, often dispensing brutal punishment to people found guilty of petty crimes or breaking the Taliban's strict interpretation of Islamic values, has become the hallmark of areas where insurgents enjoy a high degree of influence.

A recent report by a human rights group said the Taliban systematically terrorise civilians with threatening "night letters", executions and limb amputations in order to force communities not to support the government.

Taliban commanders are even issued with manuals telling them what techniques to use and who to target.

Officials say the couple's home district of Khash Rod is under almost complete Taliban control.

Sadiq Chakhansor, the head of the provincial council, said he thought the couple were intending to flee to Iran, where many young people from the region grew up as refugees, enjoying a relatively liberal environment before returning to their much more restrictive homeland.

Although the provincial governor, chief of police and leader of the local council all claimed Taliban gunmen were responsible for the murders, a spokesman for the movement denied Taliban involvement.

"I have contacted our fighters in the area and I can say that none of them were involved," said Qari Yousuf Ahmadi. "But it was a very bad thing for these people to escape from their homes without permission and it is right that they should be punished according to Sharia law."

There are almost no Afghan or foreign troops in Khash Rod, which is seen as a resting area and a passageway for militants moving through into neighbouring Helmand province where most of Britain's effort in the country is directed.