BOGOTA (Reuters) - Suspected leftist rebels opened fire on Saturday on helicopters taking a top U.S. official and Colombia's defense and police chiefs on an aerial inspection of a drug crop eradication program, authorities said. Col. Leonardo Gallego, head of the anti-narcotics division of the National Police, said Revolutionary Armed Forces of Colombia (FARC) rebels were believed responsible for the incident, which occurred over opium poppy fields in southwest Huila province. It was not immediately clear if any of the police choppers were hit by gunfire in Saturday's shooting and U.S. embassy officials were unavailable for comment. Randy Beers, the U.S. assistant secretary of state for international narcotics and crime, was aboard one of six police helicopters that came under fire, Gallego said. Others in the air convoy included Colombian Defense Minister Gilberto Echeverri, National Police chief Gen. Rosso Jose Serrano and Juan Carlos Esguerra, Colombia's ambassador to Washington. The FARC, Latin America's largest and oldest guerrilla group, has long been accused of providing protection for clandestine drug laboratories and plantations of opium and coca leaf, the raw material for cocaine. Colombia is the world's largest producer of cocaine and an increasingly prominent source of heroin sold on U.S. streets. Its U.S.-backed drug crop eradication program is among the most ambitious anywhere in the world. But the U.S. embassy recently reported a sharp increase in Colombia's coca cultivation.