U.S. cargo ship captain detained in Venezuela - embassy

* Three rifles found on board, U.S. official says

* Captain and 14 crew members are American citizens

* Latest incident in tortuous U.S.-Venezuelan ties

By Paulo Prada

CARACAS, Sept 6 (Reuters) - Venezuelan authorities have arrested the American captain of a U.S.-flagged cargo ship after finding three rifles on board, an embassy official said.

The detention follows last month's arrest of another U.S. citizen who entered illegally from neighboring Colombia and was denounced as a possible "mercenary" by President Hugo Chavez.

Socialist Chavez, who seeks re-election next month, is a ferocious critic of Washington, and his nearly 14-year rule has been characterized by frequent bilateral spats and incidents.

The U.S. Embassy official in Caracas said the "Ocean Atlas" has been in Maracaibo port, in west Venezuela, since last week, with the captain detained "a few days ago."

"It's an evolving situation," he added, saying it was unclear if charges were being pressed against the captain. He did not provide the captain's name.

Officials at Venezuela's interior and ports ministries did not immediately respond to questions about the incident.

A spokeswoman for Intermarine LLC, the New Orleans-based company that operates the vessel, could not immediately provide any details either.

The embassy official said the captain and 14 crew members, who remain on the vessel, are American citizens. The rifles, he said, were listed on the ship's manifest, but it was unclear whether their presence violated any local law.

Weapons are common on commercial ships and are often deemed necessary as possible defense against pirates or other threats.

The Ocean Atlas, according to Intermarine's website, is a heavy-lift, multipurpose cargo vessel built in 2000 with a length of 120 meters (394 feet).