US prepares for biggest-ever Agent Orange cleanup in Vietnam

More than four decades after the Vietnam War ended in 1975, ties between the United States and Vietnam are less seen through the prism of the conflict and more through shared concerns over China.

Vietnam has emerged as the most vocal opponent of China's territorial claims in the South China Sea and has been buying U.S. military hardware, including an armed, Hamilton-class Coast Guard cutter.

The United States, in turn, accuses China of militarising the strategic waterway, through which more than $3 trillion in cargo passes every year, and sees Vietnam as a crucial ally in drawing regional opposition to Beijing's behavior.

But U.S. officials including Mattis - who is on his second trip to Vietnam just this year - hope that addressing America's wartime legacies like Agent Orange can become a vehicle for further strengthening ties.

When a U.S. aircraft carrier visited Vietnam in March, for example, one of the places U.S. sailors visited was a Vietnamese shelter for people suffering from the effects of Agent Orange.

The United States just completed a five-year, $110 million program that cleaned soil contaminated by Agent Orange at Danang International Airport, which was one of the main air bases used for storing and spraying the herbicide between 1961 and 1971.

But officials from the U.S. Agency for International Development, which is overseeing the project, said the Bien Hoa site will be four times larger than Danang, a massive undertaking that is expected to cost $390 million, according to a factsheet distributed to reporters.

According to the U.S. Congressional Research Service (CRS), one soil sample from Bien Hoa had a "toxic equivalency," or TEQ, of more than 1,000 times over the international limit.

A 2011 study conducted by a private consulting firm determined that contaminated soil had spread from hot spots at the base into nearby lakes, ponds, creeks, and drainage ditches, increasing the amount of soil and sediment that will require treatment.

"The impacts on the community is very difficult to measure. Dioxin has impacts (on health) at very low concentrations and they're not real consistent," one of the U.S. AID officials said, speaking on condition of anonymity.