The virus, found buried 30m down in the frozen ground, poses no danger to humans or animals, but other viruses could be unleashed as the ground becomes exposed, scientists said.

"This is the first time we've seen a virus that's still infectious after this length of time," said Professor Jean-Michel Claverie, from the National Centre of Scientific Research (CNRS) at the University of Aix-Marseille in France.

The pathogen, called Pithovirus sibericum, belongs to a class of giant viruses that were discovered 10 years ago.