Venezuela's self-appointed president Juan Guaido has tried to wrest control of the country's state oil company, shortly before the United States imposed sanctions on the firm.

Key points: As self-appointed president, Juan Guaido currently has no source of revenue

As self-appointed president, Juan Guaido currently has no source of revenue US sanctions are designed to cut off President Nicolas Maduro's access to oil funds

US sanctions are designed to cut off President Nicolas Maduro's access to oil funds Oil accounts for most of Venezuela's income in hard currency

The moves were aimed at intensifying pressure on President Nicolas Maduro, who was re-elected last year in a contest widely seen as fraudulent.

Mr Guaido, the opposition leader who proclaimed himself president last week, has ordered congress to appoint new boards of directors to state oil company PDVSA and its US subsidiary Citgo.

Without a source of revenue or control of the military, he faces difficult odds in assuming leadership of the country, despite support across most of the Western Hemisphere.

Mr Guaido, who has not yet appointed a cabinet, faces the intricate legal challenge of nominating new leadership for PDVSA and its subsidiaries, including Citgo Petroleum, who would manage the companies during a transition.

The White House's measures to freeze PDVSA's US assets — including proceeds from oil exports — and limit the company's transactions are an attempt to largely cut off Mr Maduro's access to oil revenue, which accounts for most of the country's income in hard currency.

US refineries can receive Venezuelan oil already paid for and currently at sea, Treasury Secretary Steven Mnuchin said.

"If the people in Venezuela want to continue to sell us oil, as long as the money goes into blocked accounts we will continue to take it. Otherwise, we will not be buying it," he said.

US National Security Adviser John Bolton said the sanctions would block $US7 billion ($9.8 billion) in PDVSA's assets, "plus over $11 billion in lost export proceeds over the next year".

The White House has not explained why the phrase "5,000 troops to Colombia" appeared on John Bolton's notepad. ( AP: Evan Vucci )

Venezuela exports about 500,000 barrels of oil daily to the United States, mostly to Citgo and refineries owned by Valero Energy Corp and Chevron Corp.

The country's production has dropped in recent years to about 1.4 million barrels a day because of economic mismanagement and lack of investment.

'5,000 troops to Colombia'

Mr Maduro remains in control of the country because of support from the military, despite massive protests against his Government amid an economic crisis that has caused millions to flee the country.

Many people are going hungry, while inflation has skyrocketed and left basic goods unaffordable.

Venezuelans have been protesting against President Nicolas Maduro after months of food shortages and mass emigration. ( AP: Boris Vergara )

During a press conference at the White House to announce the sanctions, Mr Bolton held the legal pad containing the phrase "5,000 troops to Colombia", which was visible to reporters.

There was no mention during the news conference of any plans regarding troops in Colombia, which borders Venezuela.

But photographers from several news outlets captured images of the phrase.

Asked to explain the words, the White House said in an email that: "As the President has said, all options are on the table."

The sanctions do not prohibit deals between PDVSA and major US oil companies operating in Venezuela, including Chevron, Halliburton Co and Schlumberger NV.

It does, however, prevent US firms from exporting certain oil products Venezuela needs to blend with its own crude for sale elsewhere.

A clause in the constitution emboldened Mr Guaido, as president of the National Assembly, to assume interim leadership. ( AP: Fernando Llano )

Attempting to redirect funds to Mr Guaido's team could give him room to move in the short term as appointing new boards at PDVSA and Citgo could take a long time, according to experts.

Frank Holder, the head of Berkeley Research Group's Latin American practice, said Mr Guaido could face difficulties in appointing a Citgo board against the wishes of its parent company.

But Mr Guaido's team may try a number of things, even if they were unlikely to work, because "politically it shows he is trying to do something, that he has power", Mr Holder said.

The United States imposed sanctions on Venezuela and PDVSA in 2017 that prevented Citgo from repatriating dividends to its parent company.

Citgo had about $US500 million in cash at the end of September, according to a creditor who spoke to Reuters, and $US900 million in available credit.

Reuters