The United States has sent several military vessels to the Caribbean Sea and launched a series of deadly naval strikes on boats in recent weeks, a campaign that experts say is illegal.

At least 27 people have been killed in the US strikes so far, with the military buildup sparking fears in Caracas that the ultimate goal is a change of government in Venezuela.

Caracas has launched military drills, including the deployment of 17,000 troops in Tachira state, which borders Colombia, according to the commander there, General Michell Valladares.

While President Donald Trump weighs whether to expand US military action from the sea onto land, the admiral overseeing the naval strikes announced he will step down.

The latest boat attack on Thursday left survivors among those on board, media outlets including CBS, CNN and NBC reported, citing unidentified US officials.

The Pentagon did not respond immediately to an AFP request for information about the number of survivors or their condition.

In a significant escalation, Trump indicated on Wednesday he had authorized covert CIA action against Venezuela and was considering further action against the alleged drug cartels.

"We are certainly looking at land now, because we've got the sea very well under control," Trump told reporters in the Oval Office.

A day later, Admiral Alvin Holsey announced he would leave his position as head of US forces operating in Central and South America.

The head of US Southern Command gave no reason for retiring just a year into his tenure, which follows multiple top US military officers being dismissed in recent months.

- Venezuela military exercises -

Venezuela's leftist leader Nicolas Maduro has decried any attempt by Washington at "regime change" and criticized "coups d'etat orchestrated by the CIA."

Its vice president, Delcy Rodriguez, also denied a report that she had negotiated with the United States to oust Maduro.

Under Maduro's direction, the Venezuelan military carried out exercises in the country's biggest shantytowns this week after another US strike.

AFP journalists saw troops deployed on Thursday in Tachira, where the three main bridges connecting Venezuela with Colombia are located.

Venezuelan forces were also stationed in southern Amazonas state, which borders Colombia and Brazil, and multiple coastal areas.

Trinidad and Tobago, located off the coast of Venezuela, is investigating whether two of those killed in US strikes at sea were its citizens, officials said on Wednesday.

Colombian President Gustavo Petro recently called on the United Nations to open a "criminal process" against Trump for the attacks, which he believes have also killed Colombians.

Washington has not released evidence to support its assertion that the targets of its strikes are drug smugglers, and experts say the summary killings are illegal even if they hit confirmed narcotics traffickers.

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ADMIRAL GROUP