US strikes in the Caribbean and the Pacific in recent weeks have killed at least 62 people on boats that Washington claims were ferrying drugs. Family members and victims' governments have said some of them were fishermen.

UN rights chief Volker Turk said these people had been killed "in circumstances that find no justification in international law".

"These attacks -- and their mounting human cost -- are unacceptable," he said in a statement.

"The US must halt such attacks and take all measures necessary to prevent the extrajudicial killing of people aboard these boats, whatever the criminal conduct alleged against them."

President Donald Trump's administration has said in a notice to Congress that the United States is engaged in "armed conflict" with Latin American drug cartels, describing them as terrorist groups as part of its justification for the strikes.

Tensions are mounting in the region with Trump saying he has authorised CIA operations in Venezuela, and that he is considering ground attacks against alleged drug cartels in the country.

"Countering the serious issue of illicit trafficking of drugs across international borders is -- as has long been agreed among states -- a law-enforcement matter, governed by the careful limits on lethal force set out in international human rights law," Turk said.

"Under international human rights law, the intentional use of lethal force is only permissible as a last resort against individuals who pose an imminent threat to life."

Turk stressed that "based on the very sparse information provided publicly by the US authorities, none of the individuals on the targeted boats appeared to pose an imminent threat to the lives of others or otherwise justified the use of lethal armed force against them under international law".

He called for "prompt, independent, and transparent investigations into these attacks".