KUNMING, June 15 -- An international gang of drug smugglers with over 50 juveniles used as drug mules has been busted in southwest China's Yunnan Province.

Of the 63 suspects arrested in the operation, more than 50 are aged between 15 and 25 years old. A total of 13.2 kg heroin was seized in the process.

Kunming railway police said 23 teenagers were detained in 10 days last August, all with drugs concealed inside their bodies. Another 32 were seized from September to January.

Investigators found all the teenagers were coerced by a gang mustered by a suspect from neighboring Sichuan Province. They were trained in Sichuan before going to border areas in Yunnan and taking drugs back to Sichuan by train.

According to the police, drug dealers have tended to recruit "cleaner" teenagers to traffick drugs in recent years. The Kunming police busted a similar gang in 2013.