The tot, in full Muslim dress, was made to pose with the weapon dubbed the "Widowmaker" as part of a sickening propaganda stunt staged by extremists of the Islamic Jihad Union, linked to al-Qaeda.
The group - who target British and US troops from lairs on the Afghan border with Pakistan - also pictured the girl and five other toddlers with heavy weapons in front of a black Islamic flag.
Three boys aged five or six wield rifles in another chilling image posted online.
Tory MP and former soldier Patrick Mercer said: "This is a most immoral use of young children."