BY JUAN O. TAMAYO

 PUERTO OSPINA, Colombia -- Two civilian spies pulled hoods over their faces
 and marched into the jungle, guiding 123 heavily armed Colombian Marines on a
 seven-hour raid against coca-producing centers.

 The mission proved rewarding for the troops from the U.S.-trained Battalion 90,
 who slogged through shin-deep mud and drenching rains to torch three drug
 ``laboratories and locate some 220 acres of coca bushes.

 But it also revealed the massive hurdles that the Marines face in the southern
 state of Putumayo, where Colombian officials estimate the acreage planted with
 coca adds up to one-third of the nation's total. Colombia's coca labs and
 plantations produce 70 percent of the cocaine that is sold on U.S. streets.

 Shortages of fuel and radios severely limit the soldiers' patrols and add to their
 danger. They lack helicopters to fumigate coca fields and to move troops
 effectively against leftist guerrillas and drug traffickers.

 Yet they are responsible for 13,130 square miles of some of the most
 impenetrable jungles in Latin America and 3,100 miles of year-around navigable
 rivers, plus thousands more miles navigable only during the wet season.

 ``And all because of this damned plant, said Maj. Carlos Serna, 37, taking a
 disgusted backhand swipe at a coca bush as he led the 123-man raid Sept. 17
 near Puerto Ospina, a village on the Putumayo River.

 U.S. TRAINING

 River Combat Battalion 90 is one of five Marine units that has received U.S.
 military training and equipment to interdict river traffic in chemicals used for
 processing coca leaves into paste and eventually cocaine.

 The 1,300-strong battalion also faces well-armed leftist guerrillas from the
 Colombian Armed Revolutionary Forces, known as FARC, who tax the drug
 traffickers and use the region's rivers to move fighters and weapons.

 Colombia's government has asked for another $500 million in U.S.
 counter-narcotics assistance for military and police units over the next year.

 The latest U.S. military team to train the battalion gave noncommissioned officers
 six weeks of classes on small-boat handling and repairs, patrol tactics and map
 reading, said commander Lt. Col. Jose Muñoz Lopez.

 U.S. Ambassador Curtis W. Kamman attended the graduation ceremony Aug. 4
 at its main base in Puerto Leguizamo, a town of 8,000 on the caramel-colored
 Putumayo River, 340 miles south of Bogota, reachable only by air or water.

 Muñoz said the U.S. aid had helped his battalion so much that by mid-September
 it had passed its record for all of 1998, when it destroyed 27 coca-processing
 centers called laboratories and 35 smaller sites called kitchens.

 During two raids last week alone, the Marines torched nine labs and kitchens,
 detained nine peasants and destroyed some 30 containers of chemicals used to
 process coca leaves into paste.

 Marines quickly repaired boat motors whenever they stalled, and swung their
 machine guns alertly toward the nearest bank as their attack boats sped over the
 Putumayo, Colombia's southern border with Ecuador and Peru.

 One Western military officer who has been monitoring Battalion 90's progress
 rated it ``an aggressive and smart bunch of guys.

 But the Marines operate under huge handicaps.

 BOATS FROM THE '50S

 Critically short of gasoline and diesel fuel, the Leguizamo base can run its
 electricity generators only eight hours a day. A forward base in Monclart, 53 miles
 upriver, has received no gasoline since August.

 Leguizamo last week sent a flotilla of 1950s-era river gunboats, tugs, barges and
 small attack boats on an eight-hour run to buy fuel in Puerto Asis, some 150
 miles to the west, half of it through guerrilla-infested jungle.

 The previous refueling flotilla was hit by two rocket-propelled grenades that
 wounded five soldiers as it returned from Puerto Asis, Capt. Amaury Peniche, the
 region's Navy commander, told a group of journalists.

 Those flotillas are similar to the ones occasionally sent out to patrol the farther
 reaches of the Putumayo: an 80- to 90-foot mother ship, used as a floating
 barracks for Marines, and squads of 22-28 foot fast attack boats.

 The patrol flotillas must reduce their range of operations during the dry season,
 from October to about January, because of low water on the river, although the
 smaller attack boats can operate year around.

 Squeezing extra duty out of last week's refueling flotilla, Battalion 90's second in
 command, Maj. Serna, used it as a floating base from which to launch two days
 of raids against coca processors near the river's banks.

 RECONNAISSANCE

 The targets were pinpointed by the two civilian spies who had scouted the area 15
 days earlier. Former coca farmers from Puerto Ospina, they switched to the
 military after guerrillas stepped up their presence there this spring.

 Although the FARC has kept five to eight rebels stationed at Puerto Ospina since
 then, collecting tolls on passing boats and drug shipments, they had apparently
 fled by the time the refueling flotilla docked.

 The two spies, hooded to hide their identifies from former neighbors, led the patrol
 unerringly to two working laboratories, a key point because labs work only when
 leaves are harvested, three to four times per year.

 Only a 20-minute march out of Puerto Ospina and into the jungle, Serna started
 smelling the chemicals used in a lab to turn leaves into paste -- gasoline, cement,
 caustic soda, fertilizers and even some pesticides.

 Ten minutes later the patrol broke into a small clearing planted with coca and
 spoted a laboratory -- a one-room hut attached to an open-sided shack, about 30
 by 30 feet, used for mashing coca leaves and storing chemicals.

 A longer march through thicker jungle opened onto a bigger coca field and a larger
 laboratory, containing five 60-gallon drums of gasoline-soaked coca, empty but
 with bedding for 18 workers and a case of mandarin-flavored Gatoraid.

 Marines later spotted a smaller coca field nearby, and Serna ordered them to
 search for the kitchen he knew had to be hidden somewhere close by. They found
 the 15-by-15 foot hut not 30 feet into the jungle.

 ``A very good day, proclaimed Serna, 37, a two-year veteran of Putumayo River
 patrols whose sister and parents live in Miami. ``Just consider what more we
 could do with more fuel, more American help, maybe a helicopter.

 CONTACT LOST

 His 123-man patrol had only three portable radios -- U.S. Army units have one per
 dozen or so men -- and one's battery died soon after the column set off from the
 gunboat Riohacha, their floating barracks.

 Radio contact with the gunboat, the column's only possibility for calling in
 reinforcements or support fire from the ship's cannon if it came under heavy
 attack, was lost half a mile into the jungle.

 ``The jungle swallows communications, Serna said. So who would he call for
 help? ``The Holy Trinity, he joked.

 Without enough radios, the column became strung out over the seven-hour
 mission. Four soldiers briefly lost their way, and at one point an officer found
 himself on a narrow trail with only two other Marines.

 At each lab and kitchen, Serna had to take legal samples of each chemical, jot
 down approximate quantities and snap photographs with a pocket camera before
 he torched the huts with the processors' own gasoline.

 But as the patrol withdrew, it had to leave the coca fields intact. To uproot the
 plants by hand would have taken days -- giving guerrillas time to set up ambushes
 -- and most plants would have grown back in any case.

 Serna did log the coca's locations with a hand-held computer that uses satellites
 to determine latitude and longitude, for relay later to National Police fumigation
 helicopters. But he knew that was useless.

 `NOTHING HAPPENS'

 ``We've been reporting positions for two years and nothing happens, he said,
 because police choppers have been too busy spraying bigger plantations
 elsewhere around Colombia.

 The Marines found the labs empty of workers or owners and detained only three
 peasants who claimed to have been clearing cow pastures nearby. ``Just making
 the machete fly, my general, one of the peasants told Serna.

 A squad of soldiers aboard one helicopter could have hit the same three sites in a
 lot less time, Serna acknowledged, and perhaps arrived with enough surprise to
 arrest lab workers or owners.

 Back aboard the Riohacha, Serna and two officers then spent another hour writing
 legal reports detailing the materials they destroyed or seized, down to a quart
 bottle of sulfuric acid and a single 9mm bullet.

 Another lieutenant later came into his room to ask Serna how to account for the
 meals given to the detained peasants, because the Riohacha's kitchen was trying
 to charge them to his unit's allocation.

 ``Well, the prisoners have to eat, so I guess some of your guys are going to have
 to eat [cold, packaged] rations, Serna replied.

 The next morning, one Marine grumbling about the incessant rains, cold food and
 low salary of about $210 per month said he was considering quitting after more
 than 18 months in the battalion.

 Drug traffickers who are competing against the guerrillas for control of the coca
 trade in the area have offered him and other Battalion 90 Marines up to $555 per
 month to join their private armies, he said.

 ``They know we had American trainers, said the Marine, ``so they think we're the
 best.