Arlington, Virginia - The Pentagon's independent watchdog is probing the legality of the US military's series of attacks on boats off Latin America that have killed at least 192 people.

President Donald Trump's administration began striking vessels in the Caribbean and eastern Pacific Ocean in September last year, claiming the boats were trafficking drugs and insisting it is effectively at war with what it calls "narco-terrorists" operating out of Latin America.

But legal experts and rights groups suggest the attacks could amount to extrajudicial killings because they have apparently targeted civilians who do not pose an immediate threat to the US.

Trump's administration has also not provided definitive evidence that the vessels it has attacked as part of Operation Southern Spear are involved in drug trafficking.

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The backlash from the operation is a blow to US Secretary of Defense Pete Hegseth, who has said that the boat strikes have been so successful that it is now hard to find targets.

"The scope of this evaluation includes the joint process for targeted vessels in the US Southern Command area of responsibility as part of Operation Southern Spear," the Pentagon's independent inspector general's office said in a statement quoted by Bloomberg.

The probe aims to determine whether the Pentagon followed the Joint Targeting Cycle – which provides six key steps for carrying out a military operation – when conducting the strikes, the agency said in a memorandum from May 11.

"We will perform the evaluation at the Pentagon and USSOUTHCOM headquarters" in Florida, it added.

The deadly strikes have marked a stark shift in the US approach to drug trafficking, which has historically focused on interdicting vessels and seizing the material.