The United States and South Korea launched a joint survey of a US army base to try to find out whether tonnes of toxic chemical were buried there decades ago. The team is using ground-penetrating radar for this purpose.
The survey followed a report by a US TV station that leftover Agent Orange, a highly toxic defoliant used in the Vietnam War, was buried in 1978 at Camp Carroll.
Some 24 US and South Korean officials and experts, escorted by Eighth US Army commander Lieutenant General John D. Johnson, scanned the camp's heliport with the radar on the first day of their survey.
No firm evidence has so far been found that Agent Orange was buried at the camp, a logistics base 216 kilometres (135 miles) southeast of Seoul.
The US military said last week that a "large number" of drums containing pesticides, herbicides and solvents were buried at Camp Carroll in 1978, although there was no specific information on Agent Orange.
It said the materials along with 40-60 tonnes of soil were removed from the site in 1979-1980 and disposed of elsewhere.
US military officials have tried to determine where the chemicals were moved to and how they were disposed of. Johnson said Thursday there was no confirmation yet that the chemicals were shipped out of South Korea.
Separately, South Korea's defence ministry has formed a task force to survey 85 former US installations that were turned over to Seoul before 2003 as part of the relocation of US troops and bases.
The US has based tens of thousands of troops in the South since the 1950-1953 Korean War, with some 28,500 US troops currently stationed in the country.
During the Vietnam War US aircraft sprayed Agent Orange and other herbicides containing potentially cancer-causing dioxin to strip trees of foliage, in a bid to deprive communist forces of cover and food.
Vietnam says three million of its people have suffered the effects of wartime herbicides.