By Julia Harte and Nate Raymond

WASHINGTON (Reuters) - U.S. prosecutors are preparing to unveil drug trafficking charges against the head of Venezuela's National Guard, according to people familiar with the case, as the United States investigates the suspected involvement of senior Venezuelan officials in the cocaine trade.

Nestor Reverol, the former head of Venezuela's anti-narcotics agency and a long-time ally of late socialist leader Hugo Chavez, is named in a sealed indictment pending in federal court in Brooklyn, New York, according to the people.

He would be one of the highest-ranking Venezuelan officials - and the only one currently in office - to face U.S. drug charges.

Reverol, who leads the branch of Venezuela's armed forces that controls the country's borders, could not be reached for comment by Reuters.

In recent years, he has rejected U.S. accusations that Venezuela has failed to curb illicit drug shipments and has touted the government’s success in cracking down on the flow of cocaine from neighboring Colombia.

The National Guard did not respond to an email seeking comment, and a National Guard press official contacted by telephone declined to comment. Venezuela's Information Ministry did not respond to an emailed request for reaction.

It is unclear what the specific charges are against Reverol, or when the charges against him will be made public.

U.S. Justice Department spokesman Peter Carr and a spokesperson for Brooklyn U.S. Attorney Robert Capers, whose office is handling the case against Reverol, declined to comment. U.S. Drug Enforcement Administration (DEA) spokesman Joseph Moses declined to comment.

U.S. prosecutors have unsealed indictments charging at least five former Venezuelan officials with drug trafficking crimes over the past four years, according to records from Florida and New York district courts.

Venezuelan President Nicolas Maduro dismisses charges of official involvement in drug trafficking as an international right-wing campaign to discredit socialism in Venezuela.

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His Socialist Party says drug interdiction efforts have improved since Venezuela expelled the DEA in 2005.

Venezuelan opposition leaders have for years accused high-level government officials of involvement in the drug trade or of turning a blind eye to the role of military officers in narcotics trafficking.

The U.S. State Department said in its annual narcotics control report this year that Venezuela has become one of the main transit routes for illegal drugs from South America due to the country's porous border with neighboring Colombia, its "weak judicial system, sporadic international counter narcotics cooperation, and permissive and corrupt environment."

Up to a quarter of all cocaine exported from South America in 2011 departed from Venezuela, according to the White House Office of National Drug Control Policy.

Members of Venezuela's National Guard have been heavily involved in the drug trade, according to Mike Vigil, the DEA's former chief of international operations.

"The National Guard has been key to opening up the doors into Venezuela for Colombian drug trafficking organizations and subversive groups," he said. "They have transformed Venezuela into a massive pipeline for cocaine into the United States and Europe."

Two other former officials with the National Guard have been indicted on U.S. drugs charges in recent years.

Relatives of Venezuelan first lady Cilia Flores have also been caught up in U.S. anti-drug-trafficking efforts.

Two of her nephews, Franqui Francisco Flores de Freitas and Efrain Antonio Campo Flores, were arrested in Haiti last month and indicted in federal court in Manhattan on cocaine trafficking charges. Lawyers for the nephews did not immediately respond to requests for comment. They have previously said the pair would plead not guilty.

U.S. officials said those arrests were not an effort to go after Maduro's government but a case of U.S. law enforcement seeking to prosecute suspected wrongdoing.

(Editing by Stuart Grudgings and Kieran Murray)