There are reasons for concern. In August, a Syrian arms trafficker was arrested in Honduras as he tried to sell missiles and other weapons to U.S. undercover agents posing as members of the Revolutionary Armed Forces of Colombia, or FARC. The sting was the third in two years in which arms traffickers were caught allegedly trying to sell missiles to U.S. informants or agents posing as Colombians. In this case, Syrian suspect Jamal Yousef was extradited to the United States, where he pleaded not guilty in New York federal court. According to the indictment, he tried to trade 17 or 18 surface-to-air missiles or SAMs; 200 assault rifles; C-4 explosives; and 2,500 hand grenades for 1 ton of cocaine offered by the undercover agents. Yousef allegedly told the agents that the arms were stored in Mexico by a relative who was a member of Hezbollah, a Lebanese militia that the U.S. classifies as a terrorist organization. An official at the Pentagon's Southern Command in Miami, who spoke on condition of anonymity because of security concerns, said the FARC is 'actively shopping' for SAMs, which have a market value of $15,000 to$110,000 each. With such weapons, the rebels could force the government to rework a strategy that has transformed Colombia's mediocre armed forces into an effective fighting force. Mobile brigades, using 50 Black Hawk helicopters given as part of the United States' $6-billion Plan Colombia aid package, have won numerous battles, forced the FARC to retreat deeper into mountain and jungle refuges and killed or captured several top FARC leaders over the last several years. With the missiles, rebels could shoot down slow-moving helicopters, as well as the propeller-driven Brazilian Super Tucano warplanes that the Colombian air force uses to attack rebel camps. Military and intelligence officials said they are mindful of the Soviet military's experience in Afghanistan, where surface-to-air missiles provided by the CIA helped mujahedeen fighters turn the tide against a large, mechanized force. After years of battlefield setbacks and thinning ranks, the Colombian rebels would need 'far more' than the 17 or 18 missiles Yousef was trying to sell to radically alter the course of the conflict, said Robert Munks, Americas analyst with Jane's Country Risk in London. 'The significance of the FARC's possession of a small number of surface-to-air missiles would force the Colombian military to change their standard operating procedures, which relies on air superiority,' Munks said. The missiles also would have a psychological and propaganda effect 'far out of proportion to the actual threat,' Munks said. Since 2002, when rebels controlled three-quarters of Colombian territory, President Alvaro Uribe's aggressive military approach has put them on the defensive. From more than 20,000 uniformed fighters, the FARC's ranks have thinned to no more than 9,000, according to official estimates. The kidnappings and drug trafficking that the group uses to finance its operations have eroded the rebels' popular support. Still, financed mainly by drug profits, the rebels remain a presence to be reckoned with, especially in rural and jungle areas. They are major clients for the world's arms traffickers, according to the United Nations Office on Drugs and Crime. (Begin optional trim) With its many armed factions, long-running wars and porous borders, Colombia is an irresistible market for arms dealers and its trafficking history has been at times colorful. Erstwhile dealers have included crooked Venezuelan and Colombian army officers who have robbed their own arsenals to resell the weapons at a big profit to Colombia's armed groups. The Colombian and U.S. governments have formed special intelligence teams to track the FARC's arms buying activities. Over the last two years, those efforts have netted major 'big fish' in the global arms bazaar. Last November, Syrian arms dealer Monzer Kassar was convicted in a U.S. federal court of conspiracy to sell millions of dollars worth of surface-to-air missiles, launchers for rocket-propelled grenades, munitions and arms training to Drug Enforcement Administration agents posing as FARCrebels. Kassar was arrested in Spain in 2007 and extradited to the United States. In April 2008, Russian arms dealer Victor Bout, the so-called 'Merchant of Death,' was arrested in Thailand allegedly trying to close a sale of SAMs, C-4 explosives, night-vision equipment and other weapons to U.S. agents he thought were Colombian rebels. He is in custody in Thailand pending a U.S. extradition request. Concern about possible missile purchases has intensified since Colombian officials said e-mails found in laptop computers recovered from the late FARC leader Raul Reyes' secret camp in Ecuador revealed that he was leading intensive efforts to acquire missiles. Reyes was killed in a March 2008 raid on his camp by Colombian forces. As described by Colombian army officials, the e-mails indicated that FARC operatives in neighboring Venezuela were in touch with officials in President Hugo Chavez's government, including the military intelligence chief and the former interior minister, to acquire missiles and other weapons. Chavez maintains that the e-mails were fabricated to discredit his government. In July, the Bogota daily newspaper El Tiempo quoted anonymous sources in a secret Colombian government agency that tracks the FARC's arms dealings as saying FARC leader Ivan Marquez was negotiating with 'Venezuelan contacts' to buy as many as 20 Russian shoulder-fired IGLA S-24 surface-to-air missiles. Hugo Chavez's government denies the accusations. Chavez has expressed sympathy for the rebels' cause but denies giving the FARC any material support. Three anti-tank missiles sold to the Venezuelan armed forces by Sweden in the late 1980s were recovered last year at an abandoned FARC camp in eastern Colombia. Chavez said they had been stolen by rebels from an army arsenal a decade ago, and not given by his government. (End optional trim) Kraul is a special correspondent. 