KUNMING, June 27 (Xinhua) -- Police in southwest China's Yunnan Province busted a large cross-border drug-trafficking gang and arrested 160 suspects during an operation that lasted over a year, local police said Thursday.

A total of nine drug dens were destroyed, over 31 kg of heroin were seized and 36 people who had been held by the drug gang were rescued, according to the railway public security department in Kunming, Yunnan provincial capital.

Local police noticed a drug trafficking ring in December 2017, and preliminary investigation found they were using the Internet to transport drugs from Myanmar. The drugs were then sold to inland areas such as Chongqing and Sichuan.

Under the unified command of the Ministry of Public Security, the railway public security bureau and the provincial public security bureau of Yunnan, local police went to seven provinces including Sichuan, Fujian and Guangdong, busting the gang in January this year.

Police said that the gang had been using the Internet to inveigle and recruit teenagers in China to transport drugs through internal concealment and has detained, extorted and even killed those who refused to do so.

Further investigation is underway.

Yunnan is a major front in China's battle against drug crime, as it borders the Golden Triangle known for its rampant drug production and trafficking.