Fentanyl use is also becoming a concern. U.S. officials said China is the main source of illicit fentanyl and fentanyl-related substances that are trafficked into the U.S., mostly by international mail. China has hit back against the accusation, saying the U.S. government can do more to reduce the drug's demand. A Foreign Ministry spokesman said the Chinese government has been playing a positive role in tackling the global drugs issue.