A third source close to the Russian contractors also told Reuters there was a contingent of them in Venezuela, but could not say when they arrived or what their role was.

Russia, which has backed Maduro's socialist government to the tune of billions of dollars, this week promised to stand by him after opposition leader Juan Guaido declared himself president with Washington's endorsement.

It was the latest international crisis to split the global superpowers, with the United States and Europe backing Guaido, and Russia and China urging non-interference.

Yevgeny Shabayev, leader of a local chapter of a paramilitary group of Cossacks with ties to Russian military contractors, said he had heard the number of Russian contractors in Venezuela may be about 400.