In response, US national security adviser John Bolton warned of "serious consequences for those who attempt to subvert democracy and harm Guaido", in a tweet that described Saab as the "illegitimate former Venezuelan Attorney General".

Several EU countries have further said that Mr Maduro has eight days to call elections and end the standoff, or else they will back Mr Guaido.

Guaidó, 35, is the National Assembly leader who took an oath of office before a crowd of anti-government protesters that held their hands up during the symbolic swearing-in in the Venezuelan capital of Caracas. Maduro's government accuses the USA and others of launching an "economic war" against Venezuela, blaming it for most of the country's problems.

It has also been revealed at least 40 people are believed to have been killed in Venezuela's recent violence, including 26 shot by pro-government forces, five killed in house raids and 11 during looting, United Nations human rights spokesman Rupert Colville said today. He said one member of Venezuela's Bolivarian national guard was reportedly killed in the state of Monagas.

He said Citgo assets in the United States will be allowed to continue to operate - provided that any funds that would otherwise go to the state-owned oil company be sent to a blocked account in the United States.

Guaido heads the nation's congress and he has been recognized as the nation's rightful leader by two dozen nations that contend the re-election of socialist President Nicolas Maduro was a sham, in part because his strongest opponents were barred from running. "We had expected the U.S. to work its way up to sanctions as a form of building..."

Russian Federation has derided the move as an attempt by the U.S. to exact regime change on Venezuela, in an emergency UN Security Council meeting.

PDVSA becomes a Specially Designated National, meaning USA citizens are generally prohibited from dealing with it. Inclusion on this list is a powerful tool with the potential to curb trade in Venezuelan oil far beyond US borders.

International Brent crude oil futures rose over 2 percent on Tuesday in reaction.

"If you do not find a place for that crude quickly, the room for maneuver will shrink and imports will be affected", said Asdrubal Oliveros, director of Caracas-based consultancy Ecoanalitica.

"The legitimate authorities of Venezuela have already labelled these sanctions illegal".

Meanwhile, Russian Deputy Finance Minister Sergei Storchak told Russian state news agencies Tuesday "there will probably be problems" for Venezuela in paying its debts.

Venezuela is very reliant on the United States for its oil revenue.

"We've seen Venezuelan official and military personnel heeding this call", he added, citing the defection this weekend of the country's military attache to Washington.

"Sanctions on Venezuela will lead to the deterioration of conditions of people's lives", Chinese Foreign Ministry spokesman Geng Shuang told a news briefing in Beijing.