Washington DC - President Donald Trump on Tuesday said three boats in total from Venezuela had been "knocked off," a day after he confirmed a second deadly strike on what the US claims are drug traffickers in Caribbean waters.

"We knocked off actually three boats not two, but you saw two," he told reporters at the White House before heading to the UK for a state visit.

Trump did not explain on what had happened with the third boat, or if any more people had been killed.

On Monday evening, he had announced that US forces had struck a second boat in international waters, killing three people, which he described as "narco-terrorists."

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A first attack earlier this month killed 11 people and has since been described as an extrajudicial killing that violated international law, and even as murder.

Trump posted graphic videos of the two previously known strikes and his administration claims it has irrefutable evidence the people killed were traffickers seeking to ship deadly drugs to the US.

It has not, however, provided details to back up those claims, while drug trafficking itself is not a capital offense under US law.

The attacks also comes amid spiraling tensions in the Caribbean as a large US naval build-up sparks speculation that the US may be seeking regime change in Venezuela, whose President Nicolás Maduro has vowed to defend his country in the face of any aggression.

"Stop sending drugs into the United States," Trump said, in response to a reporter who asked him what message he wanted to send to Maduro.

There is no evidence suggesting the Venezuelan government is involved in smuggling drugs into the US.