A pack of stray dogs have killed 13 rare zoo animals in Bulgaria.

The animals - consisting of five deer and eight rare sheep, called mouflon - were butchered to death by a group of wild dogs, who jumped over the fences at the Sofia zoo.

According to zoo director Ivan Ivanov, a further two deer survived the attack.

He believes the homeless pooches were forced to attack because they were hungry and cold - as temperatures in the area dropped to minus 15 degrees.

Fences along the animal park perimeter have now been reinforced, and security guards have been issued with gas pistols in case the animals strike again.