Russia has opened a centre to train military helicopter pilots in Venezuela in a further sign of Vladimir Putin's support for the Maduro regime.

The news comes days after Moscow sent two planes of military personnel and equipment to Venezuela.

Elliott Abrams, the US envoy on Washington, said they 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.”

On Tuesday a spokesman for the US state department said that the issue of Russian troops in Venezuela would be raised during this week's meeting of Nato foreign ministers in Washington DC.