BEIJING (Reuters) - China's narcotics control agency has warned against the manufacture and sale of substances that can be used to make illegal drugs, a day after President Xi Jinping said China would stem the export of items used to make the opioid fentanyl. In a circular issued on Thursday, the office of the China National Narcotics Control Commission also cautioned against the risk of running into the "long-arm jurisdiction" of foreign law enforcement agencies. The United States has long sought China's cooperation to stop an illicit flow of precursor chemicals used to make fentanyl, which is up to 50 times stronger than heroin and is increasingly mixed with other drugs, often with lethal results. Under the agreement, China will go directly after specific chemical companies that make fentanyl precursors, a senior U.S. official said. U.S. officials say small chemical businesses in China make precursor chemicals that are shipped to Mexico to produce illicit fentanyl.

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Persons: Xi Jinping, Joe Biden, Ryan Woo, Miral Fahmy Organizations: China National Narcotics Control, United Locations: BEIJING, China, United States, San Francisco, U.S, Mexico