An oil tanker seized by American forces off the Venezuelan coast will be brought to a port in the United States, the White House said on Thursday, as fears mount of open conflict between the two countries.

Washington took control of the tanker in a dramatic raid that saw US forces rope down from a helicopter onto the vessel in an operation that Homeland Security Secretary Kristi Noem said was aimed at leftist Venezuelan leader Nicolas Maduro's "regime."

Maduro on Thursday responded by calling the US seizure an act of "naval piracy" that escalated tensions between Washington and Caracas.

"They kidnapped the crew, stole the ship and have inaugurated a new era, the era of criminal naval piracy in the Caribbean," Maduro said at a presidential event, adding "Venezuela will secure all ships to guarantee the free trade of its oil around the world."

President Donald Trump's administration has been piling pressure on Venezuela for months with a major naval build-up in the region that has been accompanied by strikes on alleged drug-trafficking boats that have killed close to 90 people.

"The vessel will go to a US port and the United States does intend to seize the oil," White House Press Secretary Karoline Leavitt told journalists of the tanker.

"We're not going to stand by and watch sanctioned vessels sail the seas with black-market oil, the proceeds of which will fuel narco-terrorism of rogue and illegitimate regimes around the world."

The US has also issued new sanctions targeting Venezuela, imposing curbs on three nephews of President Maduro's wife, as well as six crude oil tankers and shipping companies linked to them.

The US Treasury Department, in a statement, said it imposed sanctions on six shipping companies moving Venezuelan oil, as well as six crude oil tankers that it said "have engaged in deceptive and unsafe shipping practices and continue to provide financial resources that fuel Maduro’s corrupt narco-terrorist regime."

Four of the tankers, including the 2002-built H Constance and the 2003-built Lattafa, are Panama-flagged, with the other two flagged by the Cook Islands and Hong Kong.

The targeted vessels are supertankers that recently loaded crude in Venezuela, according to state oil company PDVSA's internal shipping documents.

Franqui Flores and Efrain Antonio Campo Flores, nephews of Venezuelan first lady Cilia Flores, were also hit with sanctions.

The two were dubbed the "narco nephews" after their arrest in Haiti in 2015 in a US Drug Enforcement Administration sting operation.

They were convicted in 2016 on charges that they tried to carry out a multimillion-dollar cocaine deal and sentenced to 18 years in prison, but were released in a 2022 prison swap with Venezuela.

A third nephew, Carlos Erik Malpica Flores, who the US says was involved in a corruption plot at the state oil company, was also sanctioned.

UN Secretary-General Antonio Guterres on Thursday expressed concern over the escalating tensions and urged restraint.

"We are calling on all actors to refrain from action that could further escalate bilateral tensions and destabilize Venezuela and the region," his spokesperson said.

Dick Durbin, the top Democrat on the Senate Judiciary Committee, said on Thursday he questioned the legality of the tanker seizure and that "any president, before he engages in an act of war, has to have the authorisation of the American people through Congress."

"This president is preparing for an invasion of Venezuela, simply said. And if the American people are in favour of that, I'd be surprised," Durbin told CNN.

Washington has accused Maduro of leading the alleged "Cartel of the Suns," which it declared a "narco-terrorist" organization last month, and has offered a US$50 million reward for information leading to his capture. (Agencies)