"The damage inflicted by Agent Orange is much worse than anybody thought at the end of the war," said Professor Nguyen Trong Nhan, the vice-president of the Vietnam Victims of Agent Orange Association (VAVA).

Between 1962 and 1970, millions of gallons of Agent Orange were sprayed across parts of Vietnam.

Professor Nhan, the former president of the Vietnamese Red Cross, denounced the action as "a massive violation of human rights of the civilian population, and a weapon of mass destruction".

But since the end of the Vietnam War, Washington has denied any moral or legal responsibility for the toxic legacy said to have been caused by Agent Orange in Vietnam.

The unresolved legacy and US denials of responsibility triggered three Vietnamese to take unprecedented legal action in January 2004.

The plaintiffs alleged war crimes against Monsanto Corporation, Dow Chemicals and eight other companies that manufactured Agent Orange and other defoliants used in Vietnam.

The case has been brought by VAVA, which was set up to promote an international campaign to gain justice and compensation for Agent Orange victims.

Preliminary hearings began in January at the US Federal Court in New York, presided over by senior judge Jack Weinstein.

Birth defects

Agent Orange was designed to defoliate the jungle and thus deny cover to Vietcong guerrillas.

It contained one of the most virulent poisons known to man, a strain of dioxin called TCCD.

First it killed the rainforest, stripping the jungle bare.

In time, the dioxin then spread its toxic reach to the food chain - which some say led to a proliferation of birth deformities.

In a small commune in the heavily sprayed Cu Chi district, the family of 21-year-old Tran Anh Kiet struggles with the problems of daily living.

His feet, hands and limbs are twisted and deformed. He writhes in evident frustration, and his attempts at speech are confined to plaintive and pitiful grunts.

Kiet has to be spoon-fed. He is an adult stuck inside the stunted body of a 15-year-old, with a mental age of around six.

He is what the local villagers refer to as an Agent Orange baby.

In Vietnam, there are 150,000 other children like him, whose birth defects - according to Vietnamese Red Cross records - can be readily traced back to their parents' exposure to Agent Orange during the war, or the consumption of dioxin-contaminated food and water since 1975.

VAVA estimates that three million Vietnamese were exposed to the chemical during the war, and at least one million suffer serious health problems today.

Some are war veterans, who were exposed to the chemical clouds. Many are farmers who lived off land that was sprayed. Others are a second and third generation, affected by their parents' exposure.

Some of these victims live in the vicinity of former US military bases such as Bien Hoa, where Agent Orange was stored in large quantities.

Dr Arnold Schecter, a leading expert in dioxin contamination in the US, sampled the soil there in 2003, and found it contained TCCD levels that were 180 million times above the safe level set by the US environmental protection agency.

Calls for US help

Professor Nhan is sadly disappointed by the US response to calls to help Vietnamese sufferers.

"Vietnam can't solve the problem on its own. Hanoi helped the US military to track down remains of MIAs (US servicemen missing in action), and we asked them to reciprocate with humanitarian aid for victims of Agent Orange," he said.

Around 10,000 US war veterans who were exposed to Agent Orange receive disability benefits for various types of cancer and other serious health problems that have been linked to dioxin.

"American victims of Agent Orange will get up to $1500 a month. However most Vietnamese families affected receive around 80,000 Dong a month (just over $5 dollars) in government support for each disabled child," Professor Nhan said.

When former US President Bill Clinton visited Hanoi four years ago, Vietnamese president Tran Duc Long made an appeal to the US "to acknowledge its responsibility to de-mine, detoxify former military bases and provide assistance to Agent Orange victims".

But Washington offered nothing beyond funding scientific conferences and further research.

Chuck Searcy, vice-president of the Vietnam Veterans Memorial Fund based in Hanoi, said: "I am baffled that the US has not offered even a small gesture of cooperation and assistance to the Vietnamese, beyond the endless talk about scientific research. Such a step would eliminate any talk of war crimes liability, or victim lawsuits."

The Vietnamese legal battle against formidable US corporate opponents is being heard in the same court as previous action by American war veterans.

It accuses the US companies of knowingly permitting Agent Orange to be sprayed for military purposes, in violation of the 1925 Geneva Protocol banning the use of chemical and biological agents.

But the legal teams representing Monsanto and other US companies are hoping to stop the case going to trial.