A Seoul appellate court, in a retrial, ruled Friday against South Korean Vietnam War veterans demanding compensation for their exposure to defoliant.

Ruling in favor of two U.S. producers of toxic chemicals, the Seoul High Court did not recognize epidemiological correlations between the defoliant and diseases such as lymphoma suffered by the more than 5,000 plaintiffs.

"Agent Orange producers did not thoroughly verify harmful effects of dioxin on human beings," the court said in its ruling. 

The court, however, said it cannot be concluded that the exposure to the defoliant directly caused such diseases. "The causes of these diseases are very complicated," the court said.

A total of 16,579 South Korean veterans and their families filed two separate lawsuits in 1999 against Dow Chemical Co. and Monsanto, seeking more than 5 trillion won ($4.9 billion) in damages, but a district court ruled against the plaintiffs.

In 2006, the Seoul High Court, in a landmark ruling, overturned the decision and ordered the two firms to pay compensation ranging from 6 million won to 46 million won to the veterans for physical handicaps they sustained from the defoliant.

It marked the first time for a South Korean court to rule against the U.S. defoliant manufacturers responsible for using Agent Orange in Vietnam in the 1960s and 1970s in order to deny the People's Army of Vietnam refuge in the jungle terrain.

The Supreme Court, however, in 2013 reversed the lower court ruling and sent the case back for review, excluding a favorable ruling on 39 soldiers who have suffered from chemical acne.

Upholding the lower court's decision for compensation, the Supreme Court recognized the link between their exposure to the defoliant and the disease for the first time in the world.

More than 4.7 million Vietnamese are said to continue to suffer from a range of illnesses, including birth defects, cardiovascular disease, cancer and nervous disorders because of the chemical defoliant dropped during the war. South Korea fought alongside the U.S against communist North Vietnam in the war.

South Korea dispatched about 320,000 soldiers to Vietnam to become the largest foreign contingent of U.S. allies fighting in the war, with 5,000 killed in action and nearly 11,000 others wounded, according to official government data.

South Korean activists estimate the number of Korean victims of the chemicals at around 150,000.  (Yonhap)