Danang airport is one of three "hot spots" identified by scientists as having dioxin levels hundreds of times higher than would be accepted elsewhere. The others are Phu Cat in south-central Binh Dinh province and Bien Hoa in southern Dong Nai province.

"The rainwater is no longer taking dioxin from that soil and putting it into the lake where people are fishing," Walter Isaacson, president and CEO of the Aspen Institute, told a news conference in Hanoi.

Isaacson, a member of the US-Vietnam Dialogue Group on Agent Orange/Dioxin which visited the site on Thursday, described a system of channels, filters and sediment tanks, and a carbon filter that takes dioxin out of the water.

The compound of dioxin is a component of "agent orange" toxic herbicides sprayed during the 1960s and 1970s war.

Its impact on human health is controversial. The US maintains there is no scientifically proven link between the wartime spraying and the more than 3 million people Vietnam says were disabled by dioxin over three generations.

HIGHLY TOXIC

Vietnamese Lt. Gen. Phung Khac Dang said that besides a thick layer of concrete being put over the north end of the base, the Ministry of Defence has built a wall and watchtowers to deter people from using the lake.

"As long as the dioxin is still there it is dangerous to us with unpredictable consequences," Dang said.

The US Congress last year appropriated US$3 million for cleanup and treatment of dioxin-related illnesses. Members of the Dialogue Group, funded by the US-based Ford Foundation, said they expected the money to be released this year.

The group's tasks include supporting clean-up at former US bases and health programmes in surrounding communities; treatment and education centres for victims; development of a Vietnamese laboratory for dioxin testing; training courses for restoration of land affected with herbicides and advocacy efforts.

Washington last year donated US$400,000 to Vietnam toward the clean-up at Danang.

The toxins left behind by the war are a thorn in otherwise friendly ties built up around trade and business since 1995.

Vietnamese diplomat Madame Ton Nu Thi Ninh told reporters that the efforts were "a work of the present and the future" to enhance relations between former enemies.

"I'm a great believer that there can be no full normalisation between the United States and Vietnam if we do not find reasonable and peaceable ways of jointly addressing this major legacy of the war," Ninh said.