Venezuela's opposition leader Juan Guaidó says he has held secret meetings with the military to win support for ousting President Nicolás Maduro.
Mr Guaidó declared himself interim president earlier this month and was immediately recognised by the US and several Latin American countries.
Major powers Russia and China back Mr Maduro and military support is seen as crucial to his hold on power.
The crisis comes as Mr Maduro began a second term after controversial polls.
Many opposition candidates were barred from running or jailed.
About three million people have fled Venezuela amid acute economic problems, and there has been an upsurge in violence in recent weeks.
US President Donald Trump tweeted on Wednesday that he had spoken to Mr Guaidó and supported his "historic assumption of the presidency", writing in a second tweet that "The fight for freedom has begun!"
UK Foreign Secretary Jeremy Hunt meanwhile is expected to urge EU nations to impose sanctions on key figures in Mr Maduro's government on Thursday, after also speaking to Mr Guaidó on Wednesday.
"We have had clandestine meetings with members of the armed forces and the security forces," Mr Guaidó writes in an article in the New York Times.
"The military's withdrawal of support from Mr Maduro is crucial to enabling a change in government, and the majority of those in service agree that the country's recent travails are untenable."
The article also says the opposition has offered an amnesty to armed forces "found not guilty of crimes against humanity".
However Mr Guaidó did not say who in the military he had been speaking with or what their positions were.
Venezuela's top military representative to the US Col José Luis Silva has defected – but senior military figures in Venezuela have supported Mr Maduro.
As head of Venezuela's National Assembly, Mr Guaidó says the constitution allows him to assume power temporarily when the president is deemed illegitimate.
Venezuela's Supreme Court has banned the opposition leader from leaving the country, however, and frozen his bank accounts.
Mr Guaidó's piece comes the same day as fresh protests began against Mr Maduro.
Venezuela's president earlier told Russian news agency RIA he was prepared to hold talks with the opposition "so that we could talk for the good of Venezuela".
He added that he was not prepared to accept ultimatums or blackmail, and insisted that he has the backing of Venezuela's military, accusing deserters of conspiring to plot a coup.
The armed forces have played a key role in supporting his government, with many officers holding posts as ministers or other influential positions.
Analysts say some members of the military may also be reluctant to switch sides for fear being held to account for alleged corruption and rights violations despite the offer of amnesty.
The US and more than 20 other nations have backed Mr Guaidó.
On Monday, the US imposed curbs on the country's state-owned oil firm PDVSA.
US National Security Adviser John Bolton spoke to businesses via Twitter on Wednesday, telling them not to deal in "gold, oil, or other Venezuelan commodities" stolen from the people by "the Maduro mafia".
BBC
