BEIJING — A top Chinese drug enforcement official says the U.S. should cut down demand for the drugs fueling its opioid epidemic rather than stressing unsubstantiated claims that China is the major source of these chemicals.

China and the U.S. have worked to build a close working relationship to fight global flows of illicit synthetic drugs, but their collaboration remains fraught.

Yu Haibin of the China National Narcotics Control Commission said Thursday there was little evidence showing China was the source of much of the chemicals used in the production of the powerful opioid fentanyl.

He urged the U.S. to share more data and police intelligence with Chinese authorities and said rampant over-prescription of pain medication and lax cultural attitudes toward drugs had fueled massive demand for opioids in the U.S.