A fire at one of Russia's largest public libraries has been raging for more than 17 hours, to destroy thousands of rare documents.

The Institute for Scientific Information on Social Sciences in south-west Moscow went up in flame late Friday. The emergency situations ministry said in a statement on Saturday that more than 200 firefighters have been trying to put up the blaze ever since.

Yuri Pivovarov, the director of the institute that holds 10 million documents and texts, described the fire as a disaster. He said in remarks on Russian television the extent of the damage was still unclear but the main storage which contained rare medieval Slavic texts did not seem to have been affected.

Pivarov said only a fraction of the collection has been digitalized.