A new United Nations report has called China out on its production of new kinds of synthetic drugs, which it says is taking place at a faster rate than can be controlled.

Criminals in Hong Kong are in turn smuggling them and selling them to lucrative markets worldwide, according to the report.

In a news conference on Sunday, Hong Kong police showcased their seizure of 95kg of cocaine with a street value of more than $13m, the city's largest confiscation of cocaine in a single police case since 2013.

Karen Joe-Laidler, a professor at the University of Hong Kong studying the city's drug trends, said it is not just cocaine now flooding the Hong Kong market.

"Over the past 20 years there has been a dramatic shift in the types of drugs that people use. The marketplace for drugs in Hong Kong is much more diverse now," Laidler said.

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The key reason is that China is one of the world's largest manufacturers of the chemicals needed to make medicinal and recreational drugs.

Chemists in China are creating these new synthetic drugs at an unprecedented rate, drug experts say. Law enforcement agencies are struggling to shut them down as the drugs are too new to be banned internationally.

A derivative of Fentanyl, a synthetic painkiller, is now being produced cheaply in China and with an effect authorities believe to be 100 times stronger than heroin.

"When you control one derivative of Fentanyl, another derivative comes out, which is not on the control list. Criminals are always one step ahead of law enforcement people," Tun Nay Soe, a coordinator at the UN Office on Drugs and Crime, said.

The latest drug market report by the European Union says China has now become the fastest-growing wholesaler of new synthetic drugs to Europe as well.