Opposition parties in Venezuela, whose President Nicolas Maduro is not recognized by the United States, said Tuesday they were not invited to new talks between the two governments.
On Monday, Maduro announced a US delegation had arrived in Venezuela to discuss a "bilateral agenda," expanding on talks in March the White House had said focused on American "energy security."
The United States and Venezuela severed diplomatic ties in 2019 after Maduro was re-elected in 2018 to a second term in a ballot boycotted by the opposition.
Before Washington enacted sanctions against Venezuela, the South American country exported almost all of its oil production to the United States.
Washington sent a high-level delegation to Caracas in March, just days after Russia invaded Ukraine.
Observers said the move sought to distance Caracas from ally Moscow, and to discuss an easing of US sanctions on Venezuelan oil after Russia's invasion caused a spike in global fuel prices.
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Maduro said Monday that National Assembly speaker Jorge Rodriguez was hosting a US government delegation.
This was despite Venezuela being excluded this month from the Summit of the Americas hosted by US President Joe Biden in Los Angeles.
Rodriguez is also the government's negotiator in talks with the opposition that have been at a standstill since last October.
On Tuesday, Omar Barboza, coordinator of the largest opposition bloc, told reporters "we have not been summoned (to the talks), we do not know the agenda."
In a bid to oust Maduro from power after his disputed re-election, Washington and dozens of other countries recognized opposition leader Juan Guaido as interim president and imposed a battery of sanctions on Caracas.
These prevent Venezuela from trading its crude oil -- which represented 96 percent of the country's income at the time -- on the US market.
Since then, Maduro has received support from Russia to continue exporting oil despite the sanctions.
After the talks in March, Washington announced it would ease some sanctions against Venezuela, including one linked to the oil company Chevron, to promote dialogue between Maduro's government and the opposition.
Caracas also released two Americans detained in Venezuela in what was widely seen as a goodwill gesture.