South Korea opened a second investigation in days into a report that the US military dumped toxic chemicals near the capital decades ago, a Defense Ministry official said on Wednesday, threatening to trigger an anti-American backlash.

Experts have been sent to the former US base in Bucheon, west of Seoul, to check out the claims after South Korean media reported that a US veteran had said "hundreds of gallons" of chemicals were buried there between 1963 and 1964.

The accusations could rekindle anti-American sentiment in the country, which saw big protests against the import of US beef in 2008 and over the deaths of two South Korean girls hit by a US military vehicle in 2002.

The United States has nearly 30,000 troops in South Korea, and the two countries are grappling with how to deal with North Korea’s nuclear program.

South Korea’s foreign ministry said the two countries, which are close allies, consider the issue serious, and local media called on the US to come clean on the "alarming" revelations.

"Even the slightest hint that the US military is hiding something could lead to widespread public distrust," the top selling Chosun Ilbo newspaper wrote in an editorial.

The latest revelations emerged after South Korean media this week uncovered comments made on the "Korean War Project," a website for ex-servicemen, a decade ago that "every imaginable chemical" had been dumped by US forces at the Bucheon base between 1963-64.

The base, about 20 km (12 miles) west of Seoul, was returned to South Korea in 1993 and is now used by South Korean engineering troops.

The defense ministry official, who declined to be identified, said the Environment Ministry was also investigating the chemical dumping claims.

The US military on Wednesday attempted to distance itself from the latest report. "Once the installation was returned to the ROK government, it became their responsibility," said United States Forces Korea (USFK) spokeswoman Cenethea Lofbom.

"The US-ROK Status of Forces Agreement (SOFA) does not impose any liability upon the US government for the condition of former installations after they have been returned and accepted by the ROK government," she said, referring to South Korea by the acronym of its official name, the Republic of Korea.

The latest allegations come after three ex-servicemen revealed last week that they had buried the toxic chemical defoliant Agent Orange at Camp Carroll in Chilgok, about 300 km (200 miles) southeast of the capital, in 1978.

At the weekend, the South Korean and USFK launched a joint investigation into that report.

USFK on Monday confirmed a large amount of chemicals were buried at the site but said they had been removed more than 30 years ago.

It said a review of military records showed trace elements of dioxin had been found at the site. It did not specify what chemicals.

Agent Orange was used to clear vegetation during the Vietnam War and was also used years later around demilitarized zones on the peninsula which was divided after the 1950-1953 Korean War.

The toxic herbicide is suspected of causing serious health problems, including cancer, and birth defects.