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.
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.
But the other sources spoke of small groups.
Russia's Defence Ministry and Venezuela's Information Ministry did not respond to requests for comment about the contractors. But Kremlin spokesman Dmitry Peskov said: "We have no such information."
The contractors are associated with the so-called Wagner group whose members, mostly ex-service personnel, fought clandestinely in support of Russian forces in Syria and Ukraine, according to Reuters interviews with dozens of contractors, their friends and relatives.
A person believed to work for the Wagner group did not respond to a message asking for information.
Citing contacts in a Russian state security structure, Shabayev said the contingent flew to Venezuela at the start of this week, a day or two before opposition protests started.