President Donald Trump in December ordered a "blockade" of sanctioned oil vessels heading to and from Venezuela, as he launched a campaign that has seen American forces take control of seven ships so far.

"We estimate that there's probably about 600 or 800 sanctioned dark fleet vessels... between Iran and Venezuela, China and Russia," Rear Admiral David Barata told a congressional hearing.

"It's a very small percentage" of vessels that have been seized, he said.

The "dark fleet" ships use various means to conceal their locations and identities: "They use false documents, they use false ownership. They claim the numbers of vessels that have already been destroyed," Barata said.

Washington has deployed a huge naval force in the Caribbean, striking boats it says are used for drug trafficking, seizing sanctioned tankers and carrying out a stunning operation that captured Venezuela's leftist leader Nicolas Maduro on January 3.

Among the vessels seized by the United States in recent weeks was a Russia-linked ship, which US forces captured in the North Atlantic after pursuing it from off of Venezuela's coast.

France's navy meanwhile interdicted a Russia-linked tanker in the Mediterranean last month, escorting it to a French port, after detaining another vessel with ties to Moscow in September.