“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.

“Certainly there are sleeping cells, certainly there are individuals who are waiting for instruction to hit and that is one of the biggest problem, which we have seen in Afghanistan as of late.”

In January, the Taliban said one of its fighters had infiltrated the Afghan army to carry out an attack which killed seven CIA operatives in Afghanistan.