The United States has sanctioned state-owned Venezuelan oil company Petroleos de Venezuela in a bid to ratchet up pressure on embattled President Nocolas Maduro, saying PDVSA is the one of the entities responsible for the country's "tragic decline".

Venezuela's self-declared interim president Juan Guaido, meanwhile, defied Maduro by announcing the opposition-controlled National Assembly will appoint a new board for PDVSA.

The company "has long been a vehicle for corruption," the US Treasury said on Monday, citing schemes designed to embezzle billions of dollars "for the personal gain of corrupt Venezuelan officials and businessmen".

The PDVSA is a primary source of Venezuela's income and foreign currency.

The South American country's oil production has plummeted from about 3.5 million barrels a day in 2000 to 1.2 million daily barrels, according to Venezuela's El Nacional newspaper.

Treasury Secretary Steve Mnuchin told reporters at the White House the sanctions were meant to intensify pressure on Maduro, who is in a power struggle with Guaido and under heavy international pressure to resign.

"The United States is holding accountable those responsible for Venezuela's tragic decline, and will continue to use the full suite of its diplomatic and economic tools to support interim president Juan Guaido," Mnuchin said.

National Security Adviser John Bolton joined Mnuchin at the briefing, saying more than 20 countries have followed the US decision to no longer recognize Maduro as president.

Venezuela's National Assembly, meanwhile, sought control over the economy, with its president Guaido announcing it would appoint new boards for the PDVSA and the Venezuelan-controlled US oil company Citgo.

The assembly will also gradually take control over Venezuelan assets abroad and take measures to guarantee transparency in the use of state resources, Guaido said in a statement.

The goal is to prevent Maduro and his entourage continuing "to steal the money of Venezuelans", said Guaido, who earlier called on Britain to prevent Maduro removing any of his country's gold held in the British central bank.

Earlier on Monday, Guaido called on the army to support fresh elections and a political transition. The military only needed to take "a political decision" for Maduro to be ousted, Guaido said in an interview published by the Colombian daily El Tiempo.

Guaido also told the Washington Post the opposition was in behind-the-scenes talks with the army.

The army has so far sided with Maduro. Guaido, who was elected president of the National Assembly on January 5 and declared himself the country's interim president last week, has promised an amnesty to soldiers who help parliament topple Maduro.

Earlier on Monday, Maduro's Defence Minister Vladimir Padrino vowed to defend Venezuela against "any aggression, of any nature or intensity".

Guaido has called massive protests against the government for this week.

The opposition demands on the government include allowing humanitarian aid, made possible by a $US20 million dollar pledge by the US, into the crisis-hit country.