About 4,000 people have been killed, a quarter of whom were civilians.

 

The worst fighting has been in the south and east of Afghanistan, near the border with Pakistan.

 

The area is also a centre for illegal opium poppy growing which accounts for 90 per cent of world output.

 

Nato confirmed on Sunday that a civilian helicopter, with a crew of eight, chartered to ferry supplies had crashed in Kandahar province.

 

The interior ministry said some of those on board had died, but Nato said its ground forces had not reached the wreckage and it did not know the fate of the crew.

 

The helicopter was owned by US security and logistics firm DynCorp International.