Vietnam and the United States on Tuesday launched the second phase of a dioxin clean-up in Da Nang, where millions of litres of Agent Orange were stored during the war between the countries.

The US sprayed the defoliant over large swathes of southern jungle during the American War in Vietnam to flush out Vietnamese soldiers, and Vietnamese victims' groups have long blamed the toxic residue for deformities and disease.

Though Washington has disputed the link between dioxin exposure and bad health, the US government has committed to help clean up toxic land in the Southeast Asian nation.

The countries, whose relations have warmed in recent years, on Tuesday began treating 45,000 cubic metres of soil contaminated with dioxin at Da Nang Airport, a task expected to be finished by mid-2017.

"I am encouraged by how this project continues to be a symbol of our honesty about the past, dealing with what remains and turning an issue of contention into one of collaboration," US ambassador Ted Osius said at the scene, according to a statement.

The first phase of the clean-up, which also treated 45,000 cubic metres, was completed in May.

"The long-term impact of the project will be the elimination of potential health risks associated with dioxin exposure from the site," the US embassy statement said.

Osius and Vietnam's Deputy Minister of National Defense Nguyen Chi Vinh switched on a thermal treatment system Tuesday at a ceremony in Da Nang, where they were photographed before a giant mound of covered earth.

The thermal technology heats the contaminated soil to temperatures high enough to break down dioxin into harmless compounds.

Da Nang Airbase was a key site in the defoliant programme, and much of the 80 million litres (21 million gallons) of Agent Orange used during "Operation Ranch Hand" was mixed, stored and loaded onto planes there.

The airbase is considered a "dioxin hotspot", where concentrations of toxic contaminants from Agent Orange are well above the globally-accepted maximum standard.

Victims groups say rates of cancer, birth deformities and other dioxin-related diseases are higher than the national average around the site.

The government says up to three million Vietnamese were exposed to Agent Orange, and at least 150,000 children were born with birth defects as a result.

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