By Jim Forsyth

SAN ANTONIO (Reuters) - The U.S. Federal Bureau of Investigation is looking into a claim made in a call to a Texas border town police department that a Mexican organized crime cartel has kidnapped a U.S. Border Patrol agent, officials said on Friday.

U.S. Customs and Border Protection officials added there are no reports of any of its 3,100 Border Patrol agents assigned to the Rio Grande Valley sector in south Texas being missing.

"Until we rule it out, we treat it as a true kidnapping," said FBI Special Agent Michelle Lee.

Chris Cabrera, a representative with the National Border Patrol Council, which represents Border Patrol agents, told Reuters concerns began with a phone call to police in the small town of La Joya.

"Someone called in and started out reporting some sort of activity, and the call then turned into threats against law enforcement, threats against the dispatcher, and ultimately threats against law enforcement in general," Cabrera said.

"He then claimed to have an agent that was kidnapped that he planned to kill, and he claimed to work for one of the cartels."

Cabrera said if there is a kidnapped agent, it could be someone not assigned to the Rio Grande Valley sector but in the area for a holiday visit.

There are several organized crime cartels working just across the border from Texas, smuggling drugs and dealing in weapons as well as sneaking immigrants into the United States. They are considered to be some of the most violent criminal organizations in the world.

Texas border towns have been relatively safe, with violent crime rates lower than other major Texas cities.

Cabrera says Mexican cartel members have attempted to kidnap U.S. Border Patrol agents in the past, but no successful kidnapping has occurred.

(Writing by Jon Herskovitz; Editing by Mohammad Zargham)