A US Army soldier gestures to a fellow solider as they participate in annual military drills in Yeoncheon, South Korea, April 11, 2013.  (PHOTO / AP)

SEOUL - US soldiers in South Korea had been caught smuggling drugs through military mail and distributing them here, local broadcaster MBC has reported.

The police in Pyeongtaek, about 70 km south of the capital Seoul, apprehended a South Korean woman in her 20s on charges of distributing synthetic cannabis in April after receiving intelligence from the US Forces Korea.

The investigation continued against a US serviceman who had been dating the arrested South Korean woman.

Some 20 US soldiers were caught distributing and taking synthetic cannabis in a building, located about 10 minutes' walk away from Camp Humphreys in Pyeongtaek which serves as the headquarters of the 28,500-strong USFK.

The group of US troops was accused of smuggling synthetic cannabis through military mail from the US mainland and trading directly with drug buyers here sought through a chat application

The group of US troops was accused of smuggling synthetic cannabis through military mail from the US mainland and trading directly with drug buyers here sought through a chat application.

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All of the overseas military mails are subject to X-ray inspection, but it had been hard to filter out a suspicious mail because of a lack of customs clearance personnel and a strict procedure to inspect the US military mails under the treaty between South Korea and the United States, the broadcaster noted.

The police called in and questioned the US soldier suspected of leading the drug distribution last week.

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The US soldier had already been detained at a facility of the US military unit, while some of the other US soldiers involved in the drug crime were transferred to the US mainland before the investigation.