A senior US official praised China on Tuesday for its commitment to fighting the spread of synthetic drugs, particularly the powerful opioid fentanyl which is ravaging swathes of the United States.
Fentanyl is a potent prescription painkiller which is extremely addictive, leading people to seek out illegal copycat versions. Pop star Prince died from a fentanyl overdose last year.
China remains a significant producer and supplier of illicit fentanyl and the chemicals used to make the compound.
But a senior Washington drugs official on Wednesday said Beijing had been "a tremendously effective partner" in trying to tackle the trade.
In a conference call with reporters Principal Deputy Assistant Secretary Luis Arreaga, from the State Department's Bureau of International Narcotics and Law Enforcement Affairs, singled out moves by Beijing in the last two years to control "more than 120 analogue and other psychoactive substances".
"China has been a tremendously effective partner with the United States and China’s commitment has been shown with some very specific steps to control their domestic production of chemicals that can be used for illicit drugs," he said.
An estimated 2.6 million Americans are hooked on prescription opioid painkillers like oxycodone and hydrocodone, or on heroin and fentanyl.
"Just to give you an idea of how horrific the problem is in the United States, in 2015 alone prescription and illicit opioids including fentanyl claimed the lives of over 33,000 Americans," Arreaga said.
He added that while current global controls to stop the manufacture and spread of such drugs remained "inadequate", successes were being achieved especially in efforts to control the export of precursor chemicals.
Earlier this month the UN's top drugs body moved to crack down on the two most common chemicals used to create illicit fentanyl.
On Monday China's government said its own drug problem was severe and growing, with particularly sharp rises in the abuse and production of synthetic drugs.
Chinese seizures of methamphetamine, ketamine and other synthetic drugs surged by 106 percent year-on-year in 2016, said Liu Yuejin, vice director of the China National Narcotics Control Commission.
The drugs are readily available for purchase online from manufacturers in China, who constantly tweak their formulas to keep the them one step ahead of laws that ban the products based on their chemical composition.