Washington has deployed warships to international waters off Venezuela's coast, backed by F-35 fighters sent to Puerto Rico in what it calls an anti-drug operation.

"It is an undeclared war, and you can already see how people, whether or not they are drug traffickers, have been executed in the Caribbean Sea. Executed without the right to a defense," Defense Minister Vladimir Padrino Lopez said as he attended a military exercise in response to the US "threat."

His remarks came just hours before US President Donald Trump announced another military strike on a boat, claiming three more alleged "narcoterrorists" were killed, bringing the total number of deaths in recent weeks to 17.

He did not say when the attack took place, and only specified that it occurred in the US Southern Command area of responsibility, which includes Central and South America, as well as the Caribbean.

The strikes have prompted debate over the legality of the killings, with drug trafficking itself not a capital offense under US law.

Washington has also not provided specific details to back up its claims that the boats targeted have actually been trafficking drugs.

Venezuelan Attorney General Tarek William Saab claimed that "the use of missiles and nuclear weapons to murder defenseless fishermen on a small boat are crimes against humanity that must be investigated by the UN."

The biggest US naval deployment in the Caribbean in decades has stoked fears the United States is planning to attack Venezuelan territory.

On Wednesday, Venezuela launched three days of military exercises on its Caribbean island of La Orchila in response to the perceived threat from a US flotilla of seven ships and a nuclear-powered submarine.

La Orchila is close to the area where the United States intercepted and held a Venezuelan fishing vessel for eight hours over the weekend.

- 'Imperial plan' -

Venezuelan President Nicolas Maduro, whom the United States does not recognize as legitimate and accuses of running a drug cartel, has urged citizens to join militia training to "defend the homeland."

Late Thursday, he announced that troops will provide residents of low-income neighborhoods with weapons training.

Maduro, for whom Washington has issued a $50 million bounty on drug trafficking charges, suspects the Trump administration of planning an invasion in pursuit.

Trump had said on Tuesday that US forces "knocked off" three boats crossing the Caribbean, but Washington only provided details and video footage of two of the strikes.

Maduro accused the United States of hatching "an imperial plan for regime change and to impose a US puppet government... to come and steal our oil."

He has repeatedly vowed Caracas will exercise its "legitimate right to defend itself" against US aggression.

Opposition figure Henrique Capriles, a two-time presidential candidate and staunch Maduro critic, said Friday he would not support any US invasion.

"I continue to believe that the solution is not military, but political," he said, adding that Trump's actions were counterproductive and "entrenching those in power."

He called for the release of nearly a thousand dissidents locked up under Maduro, and for the Venezuelan government to show goodwill in foreign relations.