China is believed to be one of the main manufacturers of synthetic drugs -- including opioids such as fentanyl -- which have been blamed for public health crises around the world.

Ahead of his first state visit to Beijing last year, President Donald Trump declared the US opioid crisis a national public health emergency and called for a halt to "the flood of cheap and deadly fentanyl" that was "manufactured in China".

China will seek greater cooperation with the US to stem the flow of fentanyl, and such efforts will not be contingent on "any political or economic issues," Liu Yuejin, vice director of the China National Narcotics Control Commission, told a press briefing.

"But... what's more important is for the US to do its best to reduce the huge demand and market" for the drug, he said.

"To resolve this issue you have to look outward, but more importantly, you have to look inward. If there are fewer US fentanyl users, then this (Chinese) fentanyl won't have a market."

China has stepped up efforts to stem the production of drug precursors and new psychoactive substances (NPS), chemicals which mimic the effects of illegal drugs while exploiting loopholes in anti-drug laws, he said.

But "the situation remains grim," concluded an annual report by the narcotics control commission released Monday.

Such substances are readily available for purchase online from Chinese manufacturers, who constantly tweak their formulas to keep them one step ahead of laws that ban the products based on their chemical composition.

Drug makers "increasingly turn to non-controlled chemicals to illegally produce precursors," with the import of such compounds on the rise, the report said.

As a result, new synthetic drugs are rapidly emerging, with 34 new NPS discovered last year -- accounting for 15 percent of NPS known in China.

Authorities cracked down on 388 criminal cases related to drug precursors and seized 2,384 tonnes of precursor chemicals, it said.

But "professional criminal gangs" dealing in such chemicals have emerged due to "robust demand".

In a statement ahead of Tuesday's International Day Against Drug Abuse and Illicit Trafficking, President Xi Jinping said China must "severely crack down on illegal drug crimes ... and resolutely destroy the network of drug trafficking gangs... to eradicate the spread of the drug problem."

In 2017 the number of known drug users in China rose 1.9 percent to 2.6 million, according to the report. Of these, some 60 percent consumed synthetic drugs and 38 percent used opioids such as heroin.