The tiny indigenous Kofan community of Santa Rosa de Guamuez in Colombia had it hard enough with pressures from settlers on their reservation, without Roundup Ultra containing Cosmoflux 411F, a weedkiller that is being sprayed on their villages in a concentration 100 times more powerful than is permitted in the United States.

Aurelio, a Kofan village elder, shows us around his village. The Kofan have been here 500 years. Now it looks as though their time is up. Pineapples are stunted and shriveled. The once green banana plants are no more than blackened sticks. The remains of a few maize plants can be seen here and there, but the food crops have been devastated. There is hunger at Santa Rosa. He is close to despair.

Colombian babies and children are falling ill. Peasants, already miserably poor, are getting hungrier. Indigenous tribes are being torn apart and whole communities pushed into exile.

The reason is the US-sponsored Plan Colombia, conceived by President Bill Clinton and roundly embraced by President George W Bush, designed to eliminate all cocaine production in Colombia. A key element is the spraying from planes of a highly concentrated chemical toxin on the coca bushes, whose leaves provide the raw material for the drug.

The coca bushes have generally survived. In the front line of America's war on drugs it is humans and the environment that have become the victims.

Investigations by The Observer have revealed for the first time the extent of the damage which both the Colombian and the US governments have tried to keep secret since the scheme started in late December. Against a growing mass of evidence to the contrary, they claimed last month: 'The aerial spraying did not cause any injury or significant damage to the environment.' The reality is that the results on the ground are disastrous.

The small farmers in this rich tropical valley don't believe the official accounts as they wonder how they can replace their crops and the chickens and fish that have been poisoned in their farmyards and ponds.

Meanwhile coca bushes are sprouting anew. Wherever the farmers have been able they have cut off the poisoned leaves to prevent the toxins reaching the bushes' roots and the coca is reviving. On the hills of Putumayo their lime-green leaves are holding the promise of new thrice-yearly harvests from which the narcotic will be manufactured again: their flourishing presence mocks the politicians and soldiers in Washington and Bogota.

At a village outside La Hormiga, a group of sick children are gathered by their mothers at the gates of the school whose small garden was ruined by the poison that rained on it early in the mornings on 22 December and 6 January. 'The planes came over at the height of a palm tree accompanied by helicopter gunships which circled around,' said Juana, a young teacher at the school. 'The plants the children were tending in the school garden withered and the pullets they were looking after all died.'

Like other Colombians, she did not want her real name used for fear of reprisals by government forces or their allies, the 'paracos' - the paramilitary death squads.

Children from local schools are showing signs of serious skin infections, which heal over but continually recur.

Gloria, a teacher at the school at El Placer, reports similar illness. 'About 230 of the 450 pupils at our school have gone down with diarrhea, and respiratory and constantly recurring skin infections,' she said.

Domestic animals have fared even worse. The tilapia that have brought a new prosperity to farmers who had built fish ponds are dying in their thousands as are dogs, pigs and other livestock.

Plan Colombia, promoted by the US and Colombian governments and gingerly accepted by the British and other European Union countries, is dissolving in failure, death and vast pollution of the Amazonian forest within months of its launch in December.

Under the plan, the Colombian armed forces are being given US weapons and training. These are same troops who over the decades have accumulated honors and medals for their battles with unarmed civilians and have frequently committed atrocities with Western help.

Now Colombians, disillusioned alike with politicians, the increasingly aimless guerrillas and the death squads, are becoming enraged at America's 'war on drugs' whose front line is in their villages.

Thousands have fled the Putumayo for neighboring Ecuador, adding to the estimated 2,100,000 Colombians who have been displaced within the country by war.

Those who stay - and who dare to criticize the war on drugs - complain that Washington is seeking to halt the production of cocaine and heroin while doing nothing to stop the drug trade in the US itself where the bulk of the profits are made - letting senior racketeers go free while filling US prisons with minor offenders from the ethnic minorities.

But what is scaring them most is what the chemicals are doing to them. Consignments of the poison being used in Colombia contain labels warning that it causes damage to crops, which must be 'shielded with screens from aerial spraying to prevent droplets falling on the green parts of useful plants'. The warning also says that application must be done on windless days.

The people who do the spraying in this valley do not supply screens and the peasants couldn't afford them if they could find them. Nature does not often provide windless days in the tropical Andean valleys. And the coca bushes are often planted among other crops.

The chemical, based on the compound glyphosate, is manufactured by the US Monsanto Corporation using British ingredients, hexitan esters, supplied by ICI Specialty Chemicals, and liquid isoparafins manufactured by Exxon. It damages the human digestive system, the central nervous system, the lungs and the blood's red corpuscles. Another constituent causes cancer in animals and damage to the liver and kidneys of humans.

The villagers' fears about the chemicals appear to be well founded. The World Health Organization has found that glyphosate is easily transmitted to humans through foods such as raspberries, lettuces, carrots and barley - with traces of the chemical found in crops sown a whole year after the soil had been dosed with it.

Elsa Niva, a Colombian agronomist who works with the Pesticide Action Network, ridicules the US government's claims that Roundup Ultra is safe and no more poisonous than aspirin or table salt.

She has written that in the first two months of this year local authorities have reported 4,289 humans suffering skin or gastric disorders while 178,377 creatures were killed by the spraying including cattle, horses, pigs, dogs, ducks, hens and fish.

According to Colombian NGOs, the government, backed by Washington, has done its best to discredit reports of damage from Roundup Ultra, accusing complaining peasants of being in league with the drug traffickers and guerrillas. The first Blair government adopted a similar attitude to the complaints: during and after several flying visits to Colombia, Mo Mowlam, the Minister then in charge of drug problems, belittled reports of the damage Roundup Ultra was causing. 'She kept on saying, "Where's the evidence?" when we told her of the effects of the poison,' remarked one senior member of a UK aid agency.

Human rights workers have expressed dismay at their treatment by British officials. 'One official visited me. He was very aggressive, dismissing our reports from the Putumayo of the damage done as "rubbish". I felt insulted. He was trying to intimidate me,' said one.

Luis Fernando Arango, a conservative lawyer and university teacher who opposes the spraying, said: 'Anyone who protests about this is labeled a drug dealer. Years into the future a lot of old men with dandruff will get together in Geneva and talk about it. But by then there will be no countryside left.'