The added protections are part of a directive issued in recent weeks by Gen John Allen, the top US commander in Afghanistan, to guard against insider threats, according to a senior military official.

The so-called guardian angels provide an extra layer of security, watching over the troops as they sleep, when they are exercising, and going about their day.

Among the new measures introduced, Americans are now allowed to carry weapons in several Afghan ministries. They have also been told to rearrange their office desks so they face the door.