Alexei Pushkov, chairman of the information committee at Russia's Federation Council, called Guaido's declaration "an attempted coup" orchestrated by Washington - a claim echoed by Maduro in a televised broadcast in which he said, "We've had enough interventionism here". A US official said intermediaries were used to deliver messages to Guaido's political mentor and opposition power broker Leopoldo Lopez, who is under house arrest after he tried and failed to lead a mass uprising against Maduro in 2014.
"The US government has limited ability to provide emergency services to US citizens in Venezuela", it said.
"This is as dramatic a situation as any that I can think of", said Max Cameron, a professor of political science with a focus on Latin American affairs at the University of British Columbia.
One of Reuters' two anonymous Russian sources, who the news agency said is close to the Wagner group, said the contractors first arrived in advance of the May 2018 presidential election, but another group arrived "recently".
Maduro, in a fiery speech on Wednesday, said he was cutting off diplomatic relations with the United States for instigating a "coup" against him.
The success of the uprising will depend on whether the military and the regime's intelligence apparatus continue to support Maduro, analysts say.
But Pence said Trump makes a distinction on intervening in the Americas. He made a point of referring to Maduro's presidency in the past tense.
"And yet President Trump has always had a very different view of our hemisphere".
The Russian Foreign Ministry accused Washington of totally disregarding the norms and principles of global law and attempting to play the role of "the self-proclaimed arbiter of the destinies" of other nations.
US military intervention is widely seen in Washington as an unpalatable and unlikely option.
Turkish President Tayyip Erdogan offered support for Maduro too. "His initial tweet in support received thousands of shares and likes and a later one included the lines "#WeAreMaduro" and "#YankeeGoHome".
In Pence's second call, which according to a White House official was kept under wraps for security reasons, the vice president assured Guaido: "We're praying for you, the USA stands with you, your bravery and courage are something we admire".
China, a major lender to Caracas, also voiced its support for Maduro, saying it opposed outside interference in Venezuela and supported efforts to protect its independence and stability.
Maduro started a second term on January 10 following a widely boycotted election past year that many foreign governments described as a sham.
The self-declared interim president, an industrial engineer by training, has shown no fear in challenging the socialist leader's election to a second term, in a May vote that was boycotted by the opposition and rejected by the United States, European Union and a dozen Latin American countries.
"These voices can not be ignored".
He said it was the only way to end the Maduro "dictatorship" in Venezuela, which has seen millions flee in recent years to escape sky-high inflation and food shortages.