Rosneft Trading SA, which has helped sell Venezuelan oil despite unilateral US sanctions, and Rosneft vice president Didier Casimiro were targeted by the US Treasury Department.
"As the primary broker of global deals for the sale and transport of Venezuela's crude oil, Rosneft Trading has propped up the dictatorial Maduro, enabling his repression of the Venezuelan people," Secretary of State Mike Pompeo said in a statement.
Pompeo added in a tweet: "Those who prop up the corrupt regime and enable its repression of the Venezuelan people will be held accountable."
The sanctions will block any assets of Rosneft Trading or the Belgian-born Casimiro that come through the United States. It also makes transactions with them a crime for anyone under US jurisdiction.
Opposition leader Juan Guaido, who recently returned from a global tour that included a White House meeting with President Donald Trump, hailed the new US pressure on Maduro's regime.
"Whoever supports the dictator, no matter who they are or where they come from, must bear the consequences," he said.
Russia denounced what it described as a US attempt to "bend the world to its will" as well as Washington's "banal desire to create advantages for American businesses that cannot stand up against fair competition from Russian companies on the world stage."
"The destructive US policy of sanctions is increasingly undermining global freedom of commerce, which the Americans say they defend, and raises international tensions," the Russian foreign ministry said in a statement.
Venezuelan Foreign Minister Jorge Arreaza, whose government espouses socialism, said the sanctions "violate the rights to free trade and free enterprise."
The United States and about 60 other countries recognize Guaido as Venezuela's interim president and not Maduro, who presides over a crumbling economy that has sent millions fleeing for lack of basic staples.
But US pressure has failed to oust Maduro who still enjoys the support of the Venezuelan military as well as Russia, China and Cuba.
Washington has already imposed multiple sanctions aimed at toppling Maduro's regime, particularly by cutting his government's oil revenue.
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."
Elliott Abrams, who is leading the US effort to topple Maduro, voiced hope the latest step would make a significant dent on Maduro's most important source of revenue.
"Today's sanctions are another step in the policy of pressuring the Maduro regime to allow Venezuela to escape from its terrible crisis through free and fair presidential elections," Abrams told reporters.
"There will be more steps and further pressure in the coming weeks and months," he warned.
Abrams said Rosneft handles around 70 percent of Venezuela's oil and that the country's top importer has been India, followed by China.
Delhi has promised to cut down purchases because of its strong ties with the United States.
Abrams voiced confidence that the global oil market would remain stable, pointing in part to rising US production.
"We're not trying to raise oil prices. We're trying to diminish the amount of money available to the Maduro regime," Abrams said.
The sanctions are the latest dispute between Washington and Moscow, with Pompeo speaking out forcefully against Russian support for Syrian President Bashar al-Assad's devastating assault on the rebel bastion of Idlib.
The United States in 2014 already imposed sanctions on Rosneft, one of Russia's largest companies, over Moscow's backing of separatists in Ukraine and takeover of Crimea, although the measures were limited to curbing US financing for the oil giant.
Pompeo told Russian Foreign Minister Sergei Lavrov of the impending sanctions over Venezuela when they met at the Munich Security Conference last weekend, Abrams said.
Lavrov earlier this month visited Caracas where he said US sanctions were illegal and "the main cause of the crisis in the Venezuelan economy."
