United States Defence Secretary Pete Hegseth said on Friday that he ordered a fourth strike on a small boat in the waters off Venezuela, killing four people, according to a social media post.

In his post, Hegseth said that "our intelligence, without a doubt, confirmed that this vessel was trafficking narcotics, the people onboard were narco-terrorists, and they were operating on a known narco-trafficking transit route".

The vessel was transporting "substantial amounts of narcotics, headed to America to poison our people", he added.

The strike comes less than a day after it was revealed that President Donald Trump declared drug cartels to be unlawful combatants and announced that the United States is now in an "armed conflict" with them in a memo.

The development also follows rising tensions with Venezuela, which on October 2 accused the United States of "military harassment" after Defence Minister Vladimir Padrino said five US fighter jets were detected near the country's coast.

"They are fighter jets. Imperialism has dared to approach the Venezuelan coast," Padrino said in a state television broadcast, describing the flights as "a provocation" and "a threat to the security of the nation".

He warned that any US attack would trigger a "national mobilisation", adding: "I denounce before the world the military harassment, the military threat of the United States Government against the people of Venezuela."

The standoff comes as Washington carries out its largest military build-up in the Caribbean in decades, deploying 10 F-35 aircraft to Puerto Rico last month along with eight warships and a nuclear submarine.