Arnold Schecter, a world authority on the toxic defoliant at the head of a joint US-Vietnamese research team, left Hanoi late on Monday carrying14 blood samples on his way to Germany.

US forces sprayed about 44 million gallons of Agent Orange over Vietnam between 1962 and 1971.

The BBC's Jonathan Burchill: "The legacy of Agent Orange remains"

The aim was to kill crops and prevent North-Vietnamese troops from hiding in the thick undergrowth.

Rehabilitation centres have been set up across Vietnam for deformed and retarded children of people who are alleged to have been exposed to the deadly dioxin contained in Agent Orange.

US Congress has demanded independent analysis before releasing money for research and eventual compensation.

Research blocked

Mr Schecter was a member of a 1995 US delegation whose blood samples and documents were confiscated by Vietnamese officials just before leaving the country.

Most child malformation in Vietnam is not from Agent Orange, US expert says

The documents were returned two years later and officials offered to give back the samples, but Mr Schecter said they would no longer have been of use.

Washington and Hanoi normalised relations in 1995, two decades years after the end of the war, but the issue of American use of Agent Orange remains a sore point.

Mr Schecter believes between 700,000 and several million people were exposed to the causing cancer, miscarriage and birth defects.

Milestone

"It appears most of the malformations seen in Vietnamese children are almost certainly not from dioxin or Agent Orange", Mr Schecter said on Monday.

But he described the current programme as a "major milestone" which could open the way for US government funding to help research in Vietnam.

Extensive research on Vietnam veterans exposed to the chemical has taken place in the US, leading to some compensation awards.