The Taliban have infiltrated the Afghan army and police, a recently-retired United Nations official has warned.

Taliban flag

Dr Antonio Maria Costa, former head of the UN Office on Drugs and Crime, said Taliban sleeper cells had been set up inside the security forces.

They had already carried out a number of attacks and were planning further attacks on Nato-led troops, he said.

A spokesman for the coalition forces said infiltration was a rare problem and most Afghan troops were loyal.

Dr Costa's comments come as the coalition is preparing to hand over control of the country's security to Afghan forces by 2014, the BBC's Gerry Northam reports.

Meeting the handover target in four years requires 141,000 new recruits to be found within a year - more than the current size of the Afghan army.

There are fears that the Taliban are taking the opportunity to enlist insurgents into the ranks.

"We have plenty of evidence we had a number of suicide attacks carried out by people who had been in the army, trusted because they were affiliated," Dr Costa told the BBC.