Elliott Abrams, the US envoy heading the US effort to oust President Nicolas Maduro, said that Washington believes that Moscow sent around 100 people to provide "technical assistance."

"One of the things they are doing seems to be, and we have thought this from the very beginning, helping the regime with the S-300 ground-to-air missile system which apparently got all screwed up... by the blackout," Abrams told reporters.

The S-300 is a major missile defense program designed to shoot down aircraft and other missiles that was designed by the Soviet Union. Russian official media in 2012 reported an S-300 shipment to Venezuela.

The economically crumbling country this week suffered its second nationwide blackout this month, knocking out transport, water supplies and communications and paralyzing the crucial oil sector.

John Bolton, the US national security adviser, on Friday urged Venezuela's military to "uphold its constitutional duty to protect the citizens of Venezuela."

"Maduro will only use this military support to further repress the people of Venezuela; perpetuate the economic crisis that has destroyed Venezuela's economy; and endanger regional stability," Bolton said in a statement.

The statement did not specify what exact actions the US wanted the Venezuelan military to take, but Trump has said its army chiefs could play a "vital role" in the country's future if they abandoned their support for Maduro

The statement Friday warned that any effort by "actors external to the Western Hemisphere" to deploy military assets to Venezuela with an eye to military operations would be considered "a direct threat to international peace and security in the region."

US President Donald Trump earlier called on Russia to "get out" of Venezuela.

Russia has said that its forces, which it described as military experts providing technical help, will stay "for as long as needed," calling the US effort in Venezuela an attempted coup.

Two Russian military planes landed at the main airport outside Caracas on Saturday and offloaded the equipment and troops. Secretary of State Mike Pompeo soon afterward raised objections in a phone call with Russian Foreign Minister Sergei Lavrov.

The United States and 50 other countries say that the leftist firebrand Maduro is no longer the legitimate leader of Venezuela and have recognized opposition leader Juan Guaido as interim president.