The letter, a copy of which was obtained by AFP on Thursday, is designed as a legal justification for at least three recent strikes in international waters that have killed at least 14 people.
The Trump administration has deployed several military vessels to the Caribbean Sea to counter drug smugglers amid mounting tensions with Venezuela's leftist President Nicolas Maduro.
"The president determined these cartels are non-state armed groups, designated them as terrorist organizations, and determined that their actions constitute an armed attack against the United States," the notice from the Pentagon said.
The notice also describes suspected smugglers as "unlawful combatants."
The recent US strikes targeted boats allegedly loaded with drugs off the coast of Venezuela but legal experts have raised doubts about the legality of Washington's actions.
"As we have said many times, the president acted in line with the law of armed conflict to protect our country from those trying to bring deadly poison to our shores," White House spokeswoman Anna Kelly told AFP.
"He is delivering on his promise to take on the cartels and eliminate these national security threats from murdering more Americans."
- 'Provocation' -
A White House official said the note was sent to Congress after one of the strikes on September 15, adding that it was legally mandated to do so after any attack involving the US military.
"It does not convey any new information," the official told AFP.
Tensions have mounted over the strikes and the US naval build-up.
Venezuela said Thursday it had detected "an illegal incursion" by five US fighter jets flying "75 kilometers from our shores."
Defense Minister Vladimir Padrino denounced the alleged flights as a "provocation" and a "threat to our national security."
A government statement meanwhile accused the United States of flouting international law and jeopardizing civil aviation in the Caribbean Sea.
Trump last month dispatched 10 F-35 aircraft to Puerto Rico, a US territory in the Caribbean, as part of the biggest military deployment in the area in over three decades.
He also sent eight warships and a nuclear submarine to the region as part of a stated operation to combat drug trafficking across the Caribbean to the United States.
After two Venezuelan military planes buzzed an American naval vessel last month, Trump warned Caracas that its jets would be "shot down" if there was any repeat of the incident.
Maduro has accused Trump of a covert bid to bring about regime change.
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