US Secretary of State Mike Pompeo has said that Juan Guaido, recognised by Washington as Venezuela's leader, should step aside along with President Nicolas Maduro for new elections.
In a shift after more than a year of US-led efforts to topple the leftist Mr Maduro, Mr Pompeo told reporters that the two men should set up a transitional government to arrange elections.
As recently as last Thursday, the US government indicted President Maduro and more than a dozen other top Venezuelan officials on charges of "narco-terrorism", which was seen as the latest escalation of the Trump administration's pressure campaign aimed at ousting the socialist leader.
The State Department offered a reward of up to $15m for information leading to the arrest and conviction of Mr Maduro, whose country has been convulsed by years of a deep economic crisis and political upheaval.
Meanwhile, state prosecutors in Venezuela have summoned opposition leader Guaido for an alleged "attempted coup d'etat" and attempted assassination, said Attorney General Tarek William Saab.
In a statement broadcast on state television, Mr Saab said Mr Guaido had been summoned to appear before prosecutors next Thursday following an investigation last week into the seizure of a weapons cache in neighbouring Colombia that he said was to be smuggled into Venezuela.
The subpoena was delivered to his head of security on Monday night, Mr Saab said.
The opposition leader is recognised as interim president by more than 50 countries, including the US, that refuse to recognise President Maduro after his disputed 2018 re-election.
Colombian forces last week seized a cache of weapons linked to a retired Venezuelan general wanted on drug trafficking charges in the US.
The shaven-headed general, Cliver Alcala, surrendered in Colombia and was handed over to US authorities last week.
Gen Alcala claimed ownership of the weapons discovered by police in Puerto Viejo in Colombia's Magdalena department on 23 March.
He said in social media messages the intention was to deliver them to Venezuela to "begin the liberation".
In his announcement , Mr Saab said Gen Alacala, who was once close to late president Hugo Chavez but retired when Mr Maduro took power in 2013, had been operating under "direct instructions from Mr Juan Guaido".
Mr Guaido has been repeatedly targeted by the government in a number of legal cases but never arrested, amid warnings by the US that it would be Mr Maduro's "last mistake".
Mr Guaido called on Sunday for the formation of an emergency coalition government that would be able to convince multilateral agencies to provide badly needed funding for its crumbling health system to fight the coronavirus pandemic.
The International Monetary Fund last week rejected Mr Maduro's request for a $5 billion loan, on the grounds that there was "no clarity" on international recognition of his government.