By Andrew Cawthorne

CARACAS (Reuters) - The United States expressed skepticism on Friday over Venezuela's arrest of a suspected American "mercenary," and said diplomats should be given access to the man if President Hugo Chavez's statements are true.

In the latest incident between the two ideologically opposed nations, Chavez said the American man, of Hispanic origin, was arrested crossing into Venezuela from Colombia with a notebook full of geographic coordinates.

"He has the look of a mercenary," Chavez said at an election rally late on Thursday. "This citizen wanted to enter the country illegally, for who knows what reason. He cannot say where he was going, or who was waiting for him."

The socialist Chavez is running for re-election at an October 7 vote and has been frequently invoking the possibility of violent actions by Venezuela's opposition with U.S. blessing.

That, opponents say, is grandstanding by Chavez, and especially cynical given his own history of a failed military coup against then-President Carlos Andres Perez in 1992.

A U.S. Embassy official said they still had no official information on Friday about the supposed arrest.

"We have seen the reports that Venezuela claims to have arrested a U.S. citizen. We have not been notified by the government of Venezuela about the arrest of this alleged U.S. citizen," said the official, who was not authorized to give his name.

"If in fact Venezuela has detained a U.S. citizen, we are confident that Venezuela will uphold its obligations under the Vienna convention on consular relations and grant U.S. consular office access to any detained U.S. citizen without delay."

Chavez is the United States' principal irritant in the region, and U.S. President Barack Obama's government would undoubtedly be pleased should he lose in October.

Chavez, though, is looking like a winner.

Back on the campaign trail after two bouts with cancer in a year, the president is leading his opponent, former state governor Henrique Capriles, by double digits in most polls.

According to Chavez, the arrested man had stamps in his passport from visits to Iraq, Afghanistan and Libya.

Chavez's nearly 14-year-rule of South America's top oil exporter has been punctuated by diplomatic spats with the United States. His fierce "anti-imperialist" rhetoric has played well with his power-base among Venezuela's poor majority, and made him one of the world's most controversial leaders.

(Editing by Xavier Briand)