Forests have been destroyed, birds sprayed as well as the food eaten by monkeys

Ricardo Vargas Environmentalist

"These crop dusting aircraft are spraying areas plotted with aerial photographs and are guided by satellite positioning systems."

But on the ground there is evidence that legal crops are being destroyed too.

While the fumigation campaign has been going since the end of last year, the other component of Plan Colombia, the $80m to help coca farmers switch to legal crops, has not arrived.

Aid not forthcoming

"What are we supposed to do?" said Cecilia Amaya, who heads a peasant association based in Puerto Asis, Putumayo's largest town.

"The promised help has not arrived, and we suspect it will never arrive. Corrupt politicians have already pocketed it."

Coca plant crops are just being planted elsewhere

The other concerns are the effects of the chemicals being sprayed on the environment and local residents.

Mr Mack insists that the glyphosate used in the spraying is completely safe and used by millions of Americans as a weed killer.

Polluted water supply

But in America it is not being sprayed on people tending their fields and Americans drink piped water, not from streams and lakes dusted with the chemicals as in Putumayo.

The glyphosate products sold in the US come with warning labels advising users "not to apply this product in a way that will contact workers or other persons, either directly or through drift".

And the US Environmental Protection Agency says glyphosate-based products should be handled with caution and could cause vomiting, swelling of the lungs, pneumonia, mental confusion and tissue damage.

The clinics around Putumayo all have reports of illnesses associated with the chemical spraying, particularly among children.

Breathing problems

"We are getting cases every week of some mild poisoning and the eye, skin and breathing problems which occur after the planes have passed over and dropped their loads," said a nurse at San Francisco Hospital in Puerto Asis.

Plantain crops are being killed by the fumigation too

Environmentalists have also expressed concern over the ecological cost of the US desire to destroy drug crops.

"The situation is truly alarming," said Ricardo Vargas, an environmentalist and author of a book on coca eradication.

"Forests have been destroyed... birds sprayed as well as the food eaten by monkeys, in a region with great biodiversity."

At least 10,000 peasants have fled Putumayo in the last six months, leaving behind barren fields and escalating violence that has accompanied the US-backed campaign.

Strengthening the guerrillas

Those that have stayed have sought virgin forest to fell and sow crops, among them coca; others have joined the guerrillas, strengthening the force the whole campaign is designed to undermine.

The US is attacking the Colombian peasant who makes nothing from the drug trade, whilst the huge profits are made by gringo [American] drug dealers and stashed in gringo banks

Commandante Simon Trinidad Farc spokesman

Many insist the problem is not going away, just shifting location, most immediately to the neighbouring province of Narino.

But the most obvious result of the fumigation in Putumayo is the explosion of new coca crops, not the large industrial fields that attract the crop-dusting aircraft, but small plots behind peasant shacks.

Coca growing is becoming the new cottage industry and no aerial eradication programme will be able to destroy these plantations.

Few Colombians believe the US strategy has any chance of success. The street price of cocaine has not changed since the fumigations began.

Huge demand

A kilogramme of cocaine is worth up to $50,000 in the US, $80,000 in Europe, and most Colombians believe that as long as the demand remains the supply will feed it.

For the country's largest rebel group, the Revolutionary Armed Forces of Colombia (Farc), which controls much of Putumayo and profits from the drug trade, Plan Colombia is clearly not going to work.

Commandante Simon Trinidad, a Farc spokesmen insists:

"The US is attacking the Colombian peasant who makes nothing from the drug trade, whilst the huge profits are made by gringo (American) drug dealers and stashed in gringo banks."