Venezuela struck a defiant note on Wednesday, insisting its crude oil exports were not impacted by US President Donald Trump's announcement of a potentially crippling blockade.

Trump's declaration on Tuesday marked a new escalation in his months-long campaign of military and economic pressure on Venezuelan President Nicolas Maduro.

Venezuela, which has the world's largest proven oil reserves, shrugged off the threat of more pain, insisting it was proceeding with business as usual.

"Export operations for crude and byproducts continue normally. Oil tankers linked to PDVSA operations continue to sail with full security," state oil company Petroleos de Venezuela (PDVSA) said.

Trump said on Tuesday he was imposing "A TOTAL AND COMPLETE BLOCKADE OF ALL SANCTIONED OIL TANKERS going into, and out of, Venezuela."

Referring to the heavy US military presence in the Caribbean – including the world's largest aircraft carrier – he warned "Venezuela is completely surrounded by the largest Armada ever assembled in the History of South America."

On Wednesday, he reiterated that US forces would not "let anybody go in... that shouldn't be going through," and accused Venezuela once again of taking "all of our oil."

"They took all of our energy rights, they took all of our oil, from not that long ago, and we want it back," he said, apparently referring to the nationalisation of Venezuela's oil industry.

Oil prices had surged in early trading Wednesday in London on news of the blockade, which comes a week after US forces seized a sanctioned oil tanker off Venezuela's coast.

Maduro held telephone talks with UN Secretary-General Antonio Guterres to discuss what he called the "escalation of threats" from Washington and their "implications for regional peace."

Guterres urged both sides to "exert restraint and de-escalate tensions to preserve regional stability."

Venezuela's economy, which has been in freefall over the last decade of increasingly hardline rule by Maduro, relies heavily on petroleum exports.

Trump's campaign appears aimed at undermining domestic support for Maduro, but the Venezuelan military said Wednesday it was "not intimidated" by the threats.

China's top diplomat, Foreign Minister Wang Yi, told his Venezuelan counterpart of Beijing's opposition to "unilateral bullying" on Wednesday.

"China opposes all forms of unilateral bullying and supports all countries in defending their own sovereignty and national dignity. Venezuela has the right to independently develop mutually beneficial cooperation with other countries," the Foreign Ministry said in its summary of the phone call with Yvan Gil.

Last week's seizure of the M/T Skipper, in a dramatic raid involving US personnel rappelling from a helicopter, marked a shift in Trump's offensive against Maduro.

In August, the US leader ordered the biggest military deployment in the Caribbean Sea since the 1989 US invasion of Panama – reportedly to combat drug trafficking, but taking particular aim at Venezuela, a minnow in the global drug trade.

US strikes on alleged drug-smuggling boats in the Caribbean and eastern Pacific have left at least 95 people dead since.

Caracas believes that the anti-narcotics operations are a cover for a bid to topple Maduro and steal Venezuelan oil. (AFP)