The British soldier who went missing from a base in Helmand may have been stripped naked before his death. His body, which was found in a river, was badly bruised and there were gunshot wounds to the head.

Extensive searches have so far failed to locate Highlander Scott McLaren's rifle and body armour and the cause of death will not be known until a post-mortem examination is carried out after the remains are flown back to Britain this week.

The body of the 20-year-old soldier, who was from the Sighthill district of Edinburgh, was found near Checkpoint Salang, in Nahr-e-Saraj, 17 hours after he had disappeared. The discovery was made during a second search of the area; an initial trawl of a stretch of water pointed out by local people had proved unsuccessful.

A senior Afghan police officer claimed yesterday that Highlander McLaren "had no clothes on when he was found. We do not know whether he drowned and then the Taliban shot the body, or whether he was shot and then thrown into the water."

General Sayed Maluk, of the Afghan army's 215 Corps, whose troops are being mentored by UK forces, insisted: "He was with two Afghan soldiers and they all went for a swim. But then they lost him and he must have drowned and his body taken away by the current. The Taliban have been trying to get publicity by saying they captured and killed him. They just used the body for propaganda. Local people pulled out the body and then the Taliban took it and treated it very badly, beating it and then shooting it."

However, no piles of clothes had been found to indicate that Highlander McLaren had gone for a swim. And, according to witnesses, he had left the base unaccompanied after finishing sentry duty. Initial investigations have not come up with any evidence that the soldier was suffering from any unusual psychological stress.

Highlander McLaren is the 375th member of UK forces to die in the conflict. David Cameron, who sent his condolences to his family during a visit to Kabul, announced yesterday that 500 troops would be pulled out of Afghanistan by the end of next year. An earlier plan, to withdraw the troops by next summer, was changed after consultation with military commanders.

Ministers stressed that the timetable for ending all combat operations by the end of 2014 would be adhered to and a relatively small number would stay behind to continue training Afghan forces. Defence sources have indicated that the SAS and SBS will continue their missions for a longer period.

Four Afghan civilians have been killed by a remotely operated UK Reaper drone. The aircraft was targeting insurgent leaders travelling in two trucks, but the civilians were also in the vehicles when they were hit.