Private military contractors who do secret missions for Russia flew into Venezuela in the past few days to beef up security for President Nicolas Maduro in the face of U.S.-backed opposition protests, according to two people close to them.

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.