BEIJING — U.S. assertions that China is the top source of the synthetic opioids that have killed thousands of drug users in the U.S. and Canada are unsubstantiated, Chinese officials told the Associated Press.

Both the U.S. Drug Enforcement Administration and the White House Office of National Drug Control Policy point to China as the main source of fentanyl, fentanyl analogs and fentanyl precursors that end up in North America.

Such statements “lack the support of sufficient numbers of actual, confirmed cases,” China’s National Narcotics Control Commission told DEA’s Beijing field office in a fax dated Friday.

In its letter to the DEA, which the commission also sent to AP, Chinese officials urged the U.S. to provide more evidence about China’s role as a source country.

It’s a point the state-run China Daily newspaper drove home publicly in an article this month stating that made-in-China carfentanil was not the cause of overdose deaths in the U.S.