President Donald Trump has declared that the United States is engaged in "armed conflict" with drug cartels, his administration said in a notice sent to Congress after recent strikes on boats off Venezuela.

The letter 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 organisations, 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," said White House spokeswoman Anna Kelly.

"He is delivering on his promise to take on the cartels and eliminate these national security threats from murdering more Americans."

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. (AFP)