BUCARAMANGA, Colombia (AP) _ John Fredy knows how to fire an AK-47 assault rifle, dig trenches and guard kidnap victims. But the short, skinny 13-year-old with big ears and a penetrating stare, a deserter from Colombia's largest leftist guerrilla band, cannot read or write his name.

Freezing, disoriented and on the brink of starvation, he was part of guerrilla column that was surrounded by government troops in late November as it traversed a high Andean plateau near this northern city. In lopsided fighting, at least 51 of the 360 guerrillas have died and 95 others have surrendered during a nearly monthlong assault from the troops.

Generals are calling it the army's greatest victory in nearly four decades of battle with the Revolutionary Armed Forces of Colombia, or FARC. But the episode has also exposed a sad and sinister side of this South American country's 36-year civil war: the growing use of children in combat.

It is a worldwide phenomenon. An estimated 300,000 children under the age of 18 are participating as guerrillas or government soldiers in armed conflicts in Africa, Asia and Latin America, according to the U.S.-based monitoring group Human Rights Watch.

Children from poor villagers are impressionable and easy to recruit, prized as efficient and remorseless killers. They are frequently forced to commit atrocities or sent as cannon fodder ahead of older troops, experts say.

Before the latest fighting in Colombia, some 6,000 minors were believed to belong to guerrilla factions and rival right-wing paramilitary groups here _ constituting about a fifth of the country's nearly 30,000 irregular fighters. The army only recruits soldiers who are 18 or older.

But after witnessing the fresh faces of dozens of boys and girls captured in the clashes _ not to mention the bodies of 34 youths killed in the combat _ experts fear they may have underestimated the problem.

``Either the proportion of children in the troops is far above our previous estimates or, even worse, it could mean that they are increasingly putting the children out on the front lines of the war,'' said Carel Rooy, UNICEF's chief officer in Colombia.

In interviews Tuesday at a government-supported halfway house outside Bucaramanga, captured guerrillas told how their dreams of glamor and glory in the FARC gave way to hardship and disillusionment.

A 14-year-old boy who gave only his last name, Carvajal, said he'd been lured by the swashbuckling image guerrillas had in his poor hometown in southern Meta province, a longtime FARC bastion.

``I saw the rifles they carried, and the four-by-four vehicles they drove around in, and I thought that was great, but I was kidding myself,'' he said. ``They don't pay you and they never let you see your family. You are a slave.''

Carvajal said he was recruited two years ago in Mesetas, one of five southern townships in a demilitarized zone that Colombian President Andres Pastrana ceded to the FARC in November 1998 as an incentive to start peace talks. Of the 16 boys and six girls interviewed here, more than half said they had been recruited in the zone.

Most said they joined voluntarily, but several claimed they were pressed into service. One, a 16-year-old from an Indian tribe in eastern Vichada province, said guerrillas stopped a car he was riding in with his uncle, tied him up and dragged him away, saying, ``he's a big boy now.''

According to the captured rebels, their unit left the DMZ in September on a long and grueling trek to reinforce guerrilla troops in the north. They were told to expect fighting but given little preparation. One boy said his marksmanship training consisted of firing five live rounds.

When federal troops attacked after detecting the rebels crossing a cold and barren plateau nearly 10,000 feet above sea level, discipline broke down and desperation took over.

``We had gone five days without anything to eat, and we decided to escape,'' said John Fredy, who fled with two older guerrillas, both 14. ``We didn't want to turn ourselves over (to the army), we wanted to escape and make it to the city.''

But the pressure from troops backed by helicopter gunships was too great, and on Dec. 10 the three stumbled upon an army patrol and surrendered. The army had been dropping leaflets from the air promising that deserters would not be killed.

At the halfway house, John Fredy and other former members of his unit have been given food, clothing and shelter. They are also beginning psychological treatment aimed at breaking their warrior mentality.

``They are trained to kill,'' Juan Manuel Urrutia, director of the federal government's social service agency, said this week. ``Now we must train them to love.''