The accusation was the latest in a barrage of attacks by Maduro's socialist government on the US, which Venezuelan leaders contend is behind three months of anti-government protests that have resulted in 42 deaths.
State Department spokeswoman Jen Psaki called the allegation baseless and said such claims are meant to distract Venezuelans from their own government's failure to address grave economic and social problems.
One message, dated March 23, says Kevin Whitaker, now the ambassador in Bogota but at the time the top US State Department official for the Andean region, offered his support to the opposition, which has been seeking for months to force Maduro to step down.
"Kevin Whitaker reconfirms his support and indicated new steps," according to the purported Machado email, which was addressed to a former opposition lawmaker, Gustavo Tarre. "We have a stronger checkbook than the regime to break the international security ring they've constructed giving away all of the Venezuelans' money."
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In a Twitter message, Machado called the accusations "infamy" and said she would hold her own news conference. Another email presented by Rodriguez discusses the need to "annihilate" Maduro.
"We have to clean up this rubbish, starting at the top, taking advantage of the global climate provided by Ukraine and now Thailand," said the email dated May 23.
After travelling to Washington in March to denounce Maduro's government at the Organisation of American States, Venezuela's Congress barred from serving as a legislator and accused her of treason.