US Southern Command, which oversees South America, did not reveal where the attacks over two days occurred.
Previous attacks have been in the Caribbean Sea and in the eastern Pacific Ocean.
A video of Tuesday's attack posted by Southern Command on social media shows three boats travelling in a close formation, which is unusual.
The military said they were in a convoy along known narco-trafficking routes and "had transferred narcotics between the three vessels prior to the strikes".
The military said three people were killed when the first boat was struck, while people in the other two boats jumped overboard and distanced themselves from the vessels before they were attacked.
Southern Command said it immediately notified the US Coast Guard to activate search and rescue efforts.
Southern Command's statement did not say whether those who jumped off the boats were rescued.
Calling in the Coast Guard is notable because the US military drew heavy scrutiny after its forces killed the survivors of an attack in early September with a follow-up strike to their disabled boat.
Some Democratic lawmakers and legal experts said the military committed a crime, while the Trump administration and some Republican lawmakers say the follow-up strike was legal.
US forces attacked two more boats on Wednesday, killing five people who were allegedly smuggling drugs along known trafficking routes, Southern Command said in a separate statement.
It did not provide evidence of the alleged trafficking or reveal the body of water in which the attacks occurred. Videos posted with the statement on social media showed a boat in the water and explosions.
The latest attacks bring the total number of known boat strikes to 35 and the number of people killed to at least 115 since early September, according to numbers announced by the Trump administration.
President Donald Trump has justified the attacks as a necessary escalation to stem the flow of drugs into the United States and asserted that the US is engaged in an "armed conflict" with drug cartels.
Along with the strikes, the Trump administration has built up military forces in the region as part of an escalating pressure campaign on Venezuelan President Nicolas Maduro, who has been charged with narco-terrorism in the United States.
Meanwhile, the CIA was behind a drone strike last week at a docking area believed to have been used by Venezuelan drug cartels, according to two people familiar with details of the operation who requested anonymity to discuss the classified matter.
It was the first known direct operation on Venezuelan soil since the US began strikes in September, a significant escalation in the administration's pressure campaign on Maduro's government.
With Reuters
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