Russia has opened a centre to train military helicopter pilots in Venezuela in a further sign of Vladimir Putin's support for Nicolás Maduro's regime.
It comes days after Moscow sent two planes of military personnel and equipment to Venezuela.
Elliott Abrams, the US envoy, said the US believed the teams had been flown in to repair Soviet-era S300 missile systems, which were knocked out by a series of blackouts.
But he added that the US was watching closely to see what the Russians did, adding: "We have options, and it would be a mistake for the Russians to think they have a free hand."
Rosoboronexport, the state corporation for arms sales, stated it had built the centre under a contract with Venezuelan state-owned firearms manufacturer CAVIM.
Russian-made simulators will let local pilots practise flying helicopters that Moscow has supplied to Caracas.
These include the Mil Mi-35M Hind, a gunship, the Mi-26T heavy transport helicopter and the Mi-17V-5, a transport helicopter that can be equipped with machine guns and rockets.
The centre, with Russian-trained staff, will cut the costs and time needed to train pilots, Rosoboronexport said.
Russian organisations "remain committed to deepening cooperation with government agencies of Venezuela", the statement said.
Russian-made helicopters in Venezuela are participating in "routine military activities and operations against smugglers", as well as delivery of humanitarian aid, it added, making no mention of clashes between security forces and protesters.
Rosoboronexport did not immediately reply to requests for comment.
At the opening ceremony for the helicopter centre last weekend, Vladimir Padrino López, the Venezuelan Defence Minister, said a flight simulator for the Russian-made Su-30MK2 fighter jet would be installed in another city and a factory to produce Kalashnikov assault rifles under licence would open soon after years of delays.
More than 50 countries have recognised Juan Guaidó, the opposition leader, as interim president after weeks of protests and shortages of food, water, medicine and electricity.
Maduro is struggling to contain unrest in the country amid blackouts that began on March 7 and have led to subsequent failures in water systems.
In March, 99 Russian soldiers and 35 tonnes of equipment flew in to bolster Maduro's military.
Yesterday, Alejandro Chacón, a Colombian congressman, said he received a letter from Russia's ambassador in Bogota, warning Colombia against any military action against Maduro.