US forces carried out a strike on an alleged drug-smuggling boat off the coast of Venezuela on Friday, killing four people, Defence Secretary Pete Hegseth said.

Hegseth announced the latest strike -- which brings the number of such attacks to at least four, leaving at least 21 people dead -- in a post on X.

An accompanying video showed a boat speeding across the waves before being engulfed in smoke and flames, then continuing to burn while still floating.

"Four male narco-terrorists aboard the vessel were killed in the strike," which "was conducted in international waters just off the coast of Venezuela while the vessel was transporting substantial amounts of narcotics -- headed to America to poison our people," the Pentagon chief wrote.

"These strikes will continue until the attacks on the American people are over!!!!" he wrote.

The latest military action comes after US President Donald Trump's administration said in a notice to Congress that he has declared that the United States is engaged in "armed conflict" with drug cartels.

But Washington has not released evidence to support its assertion that the targets of its strikes are drug smugglers, and experts say the summary killings are illegal even if they target confirmed narcotics traffickers.

The administration's letter, a copy of which was obtained by AFP on Thursday, was designed as a legal justification for at least three previous strikes.

"The president determined these cartels are non-state armed groups, designated them as terrorist organizations, and determined that their actions constitute an armed attack against the United States," said the notice from the Pentagon, which also described suspected smugglers as "unlawful combatants."

A White House official said the notice was sent to Congress after one of the strikes on September 15, adding that it was legally mandated to do so after any attack involving the US military. (AFP)