The United States on Tuesday announced sanctions against a subsidiary of Russian state-controlled oil giant Rosneft over its continued trade with Venezuela in defiance of Washington's attempt to break leftist President Nicolas Maduro's grip on power.

Officials in President Donald Trump's administration said Rosneft Trading SA and Rosneft vice president Didier Casimiro were targeted in the latest measures.

One official, speaking on condition of not being identified, called Rosneft Trading SA "the prime culprit" in helping Maduro escape the effect of US economic sanctions.

This is "sending a message that we will not stand idly by as foreign, extra-continental or other entities help the Maduro regime maintain and sustain its repression," he told reporters.

"Rosneft Trading has been the overwhelming provider of trading in the Maduro regime oil and has provided the overwhelming amount of financial resources and hard currency that has been coming in to the Maduro regime. Therefore this action should have a signficant impact on the Maduro regime," the official said.

Another adminstration official, who also asked not to be named, said the sanctions would hit Rosneft Trading SA business in the United States, but also send ripple effects through the company's wider activities. "The designation of Rosneft Trading and Didier Casimiro means that anyone outside the United States ... runs the risk of being sanctioned themselves," the official said.

Moscow's support for the Maduro government "is having huge consequences for them," one of the adminstration officials briefing reporters said.

Venezuela's President Nicolas Maduro has remained in power despite US sanctions and diplomatic pressure Venezuela's President Nicolas Maduro has remained in power despite US sanctions and diplomatic pressure Photo: AFP / YURI CORTEZ

The official said the United States is determined to bring down Maduro through its maximum pressure campaign, which currently is at "50 to 60 percent."

"We hope never to have to get to 100 percent of maximum pressure," the official said, responding to a question of whether the United States could ever carry out a military campaign in Venezuela.

Despite US sanctions and Washington's recognition of opposition leader Juan Guaido as the legitimate president of Venezuela, Maduro and his military backers retain a tight grip.

More than 50 countries have joined the United States in recognizing Guido, saying that Maduro's 2018 re-election was rigged.

But Maduro has survived in large part thanks to economic, military and diplomatic backing from Russia, as well as support from China and communist Cuba.

Earlier this month, Russian Foreign Minister Sergei Lavrov visited Caracas, where he said US sanctions were illegal and "the main cause of the crisis in the Venezuelan economy."

Maduro on Monday accused Trump of plotting an invasion and said "we are not afraid of military combat and we are going to guarantee peace."