A U.S. Navy sailor held in Venezuela since late last month was arrested for entering "without any type of document," the South American country's attorney general said Thursday.

An American official on Wednesday announced the sailor had been detained at a time of soaring tensions between Washington and Caracas in the aftermath of disputed elections in Venezuela, with the opposition party claiming it can prove were stolen.

In his first comments on the matter, Venezuelan Attorney General Tarek William Saab said the sailor "entered without any type of document, without any means of subsistence for what he came to do in the country."

He said the sailor held dual U.S. and Mexican nationalities.

For its part, the Pentagon said the sailor had been in Venezuela on "personal travel."

"This wasn't something that was authorized," Deputy Pentagon Press Secretary Sabrina Singh told reporters.

"The U.S. Navy is looking into this. We're working with the State Department," she said, adding that: "Of course, we'd like to see the sailor returned home."

Venezuela was rocked by protests after President Nicolas Maduro was declared the winner of a disputed July 28 election, with 25 civilians and two soldiers killed and more than 2,400 people arrested.

The opposition claims it won by a landslide, and the United States, the European Union and several Latin American countries have refused to recognize Maduro's claimed victory without seeing detailed voting results.

On Monday, Washington seized Maduro's plane in the Dominican Republic and flew it to Florida, a move the Venezuelan leader condemned as "piracy" but which Washington said was necessary due to sanctions violations.

The following day, Washington denounced an arrest warrant issued for opposition presidential candidate Edmundo Gonzalez Urrutia and warned of further action against Maduro.

The U.S. State Department has warned Americans against traveling to Venezuela for reasons including crime, unrest and wrongful detention.