-- Colombia's mammoth anti-drug campaign, backed by more than $1 billion of U.S. military and social development aid, has entered a new punitive phase of aerial spraying that is killing fields of coca as well as the legal crops of farmers here in the country's most bountiful drug-growing region.

Using U.S. and European satellite photographs to pick targets, Colombian army and police aircraft have begun spraying herbicides on small farms in western Putumayo, the southern province that accounts for more than half the country's coca production.

The flights, paid for by the U.S.-backed anti-drug campaign called Plan Colombia, have occurred almost daily over several farming communities since Dec. 22 and have wilted hundreds of acres of coca, the key ingredient in cocaine, and legal crops, which often are planted alongside coca. Local people say the chemicals have sometimes fallen on towns and farmhouses, causing people to suffer fevers. They also blame the spraying for the deaths of some cows and fish.

"Those without coca are more affected than those with it," said Hilberto Soto Vargas, a local farmer whose banana grove was fumigated even though, by his account, he pulled up his coca plants two years ago when he became a member of a Pentecostal church. "All of this is dying now," he said, pointing to his fields. "All of it."

Colombia accounts for 80 percent to 90 percent of the world's cocaine production and a growing share of its heroin. The fumigation in Putumayo marks a bold new escalation of Plan Colombia, a U.S.-backed $7.5 billion campaign to cut Colombian drug production by half in six years, by 2005.

Until recently, spraying focused almost entirely on remote industrial-sized coca and poppy plantations that grow most of Colombia's drugs. Officials claim it has denuded roughly 125,000 acres of drug fields. Now the planes are targeting more populous farming areas like this one, where coca is seen by many poor villagers as a legitimate cash crop and is often grown side by side with corn, yucca, pineapple and livestock. Often it shares a plot next to the farmer's tin-roofed shack.

The new approach is designed in part to punish several coca-rich communities that have refused to join a U.S.-backed program that pays farmers to uproot illegal crops and replace them with legal ones. Some of the communities declined to join because of threats from leftist guerrillas who profit from the drug trade.

In La Hormiga, a town 30 miles west of Putumayo's commercial center of Puerto Asis, town officials and residents say the fumigation has been devastating. In interviews, dozens of farmers said that the spray, delivered by small planes escorted by armed helicopters, has killed hundreds of acres of food crops, scores of cattle and hundreds of fish that washed up on the banks of the Guamuez River. On several occasions, several witnesses said, the aircraft dropped herbicide within the town itself.

U.S. drug control policy director Barry R. McCaffrey has said repeatedly that the herbicide, Roundup, produced by Monsanto Co., is harmless to humans and animals -- he called it "totally safe" during a November visit to Colombia.

However, in the United States it is sold with warning labels advising users to "not apply this product in a way that will contact workers or other persons, either directly or through drift." The U.S. Environmental Protection Agency says glyphosate-based products such as Roundup should be handled with caution and could cause vomiting, swelling of the lungs, pneumonia, mental confusion and tissue damage.

Several farmers here said they have experienced fever-like symptoms since being sprayed, but local doctors report only one hospitalization for chemical poisoning. Mayor Flover Edmundo Meza, whose own farm was fumigated last week, predicts widespread hunger throughout the municipality of 35,000 people because of crop damage. The loss could result in thousands of families leaving their farms, he said.

"Our intention is to eliminate these crops -- voluntarily -- and avoid these damages, but the government is not listening to us," said Meza, who took office Jan. 1. "People will not be able to eat, and we don't have the resources to address this. We are asking the government to stop at once."

The U.S. Congress has pledged $1.3 billion over the next two years to Plan Colombia, most going toward such military hardware as the helicopters used in the fumigation missions. The U.S. contribution also includes money to build small businesses, health clinics, schools and roads that Colombian officials hope will help end two decades of coca cultivation in Putumayo.

European nations have chipped in more than $200 million for social programs, but have roundly condemned the fumigation strategy. However, that approach is backed with enthusiasm by the United States; some U.S. officials in Colombia proudly display photos of denuded coca and poppy fields on their office walls.

About $81 million of the U.S. aid is available for the plan's alternative development program, which through subsidies and small loans seeks to coax farmers to abandon coca crops for legal ones. Of that sum, $30 million is marked for eradication programs that farmers must join if they are to avoid fumigation.

In December, more than 500 families signed up for crop substitution programs in Puerto Asis, an area largely protected from guerrilla forces by privately funded paramilitary groups and a nearby army base.

But not a single farmer in La Hormiga or in the neighboring municipality of San Miguel signed on to the plan when it was presented here late last summer. Gonzalo de Francisco, President Andres Pastrana's point man for Plan Colombia, said the communities understood the consequences but might have been frightened off by pressure from guerrilla forces.

De Francisco said the towns, which sent his office petitions pleading for an end to the fumigation six days after it began, will be offered another chance to sign the pacts in coming weeks. In the meantime, the spraying will continue.

"Obviously, we take these reports [of harm from spraying] seriously and we are trying to get the best information we can so we can analyze the situation correctly," de Francisco said. Fumigation is not perfect, he said, and everyone would be better off if the villagers agreed to join the programs to end coca cultivation.

The central government in Bogota argues that the spraying is necessary because as much as one-third of Colombia's coca comes from small farms like the ones here. An estimated 66,000 acres of coca are under cultivation in the municipality of Valle de Guamuez, of which La Hormiga is the capital. That is almost double the acreage of food crops and accounts for a large fraction of the province's total coca production, which has been increasing.

But a recent tour of the area suggested there is no way to fumigate from the air without harming legal agriculture as well as drug crops.

"That is the thing that hurt me," said Rosa Elvira Zambrano, a 71-year-old widow, pointing to her neighbor's four-acre coca field, which lies across a barbed-wire fence from her withering grove of banana trees and yucca. Zambrano, who has lived on a seven-acre farm inside La Hormiga's city limits for 25 years, grows food and raises chickens to support her daughter, also a widow, and three grandchildren.

On the morning of Dec. 22, she said, a group of planes and helicopters passed over her farm three times, spraying herbicide on her crops while mostly missing her neighbor's coca. "It's the government that has ruined all this," she said. "How will I eat?"

More than a dozen farmers said the aircraft appear to be spraying from high altitudes, perhaps for fear of guerrilla ground fire. The result, they say, has been indiscriminate fumigation. A reporter's inspection of fields in the area suggested that food crops have been hit at least as hard as coca.

Ismael Acosta, a 46-year-old father of five, cultivates an acre and a half of coca on his farm along the banks of the Guamuez River. He said that at noon last Wednesday, more than 10 aircraft passed over his farm, most of which is planted with corn and yucca, a common crop grown for its roots. One day later, his corn patch had turned brown and his yucca was losing leaves. A few yards away, his coca patch showed signs of yellowing.

In Puerto Asis, meanwhile, about 550 farmers are beginning a social experiment meant to end fear of fumigation. Last month, two-thirds of them signed agreements with the government to receive $1,000 payments if they pulled up their coca plants within a year. The other third, who don't grow coca, received pledges of the same subsidy as a reward for staying out of the drug business.

The farmers can keep the money or use it to buy farming supplies to get a new start with legal crops. The sum would be enough to pay for two milk cows, 50 chickens, an acre of banana trees and more.

More important, the agreements authorize the farmers to apply to a local nonprofit foundation for small-business loans from a pool of U.S. and European aid. Farmers are to get seats on the foundation's board and the chance to pitch ideas for putting such enterprises as cattle ranches and fish farms on former coca fields.

Fernando Bautista is a butcher who helps run his cousin's 15-acre coca farm along the placid Putumayo River near Santa Ana. Bautista has lost three brothers to drug-trade murders; now he says he wants to give his two daughters another way of life by starting a dairy farm with government help.

He and his cousin, Ramiro Garcia, have joined with 20 other coca farmers to pitch the idea. They plan to pool their $1,000 government payments, then seek a loan to purchase 10 cows each, build stables and buy tank trucks.

But the economics must make sense for Garcia to give up the $6,000 in annual profit he has been getting from the 35 pounds of coca paste that his farm produces each year.

Along the edge of his field stands a warning: a small patch of brilliant green plants resembling clover -- infant coca bushes, enough to plant 25 acres.

"If the government helps us, I will sell them or just pull them up," Garcia said. "If not, I'll plant them."

Rosa Elvira Zambrano, 71, walks in the field that she says the Colombian government ruined with herbicide, though she does not grow coca.