As President Donald Trump began his lengthy trip through Asia, Chinese authorities sought to dismiss allegations that their country is the primary source of the synthetic opioid fentanyl, which has been linked to more than 20,000 overdose deaths in the United States in 2016.

At a press conference highlighting joint efforts between the Chinese and U.S. to fight drug-related crime, Wei Xiaojun, deputy secretary-general of China's National Narcotics Control Commission, said that while he agrees that some fentanyl substances that have been found in the U.S. have come from his country, his agency did not see "sufficient evidence… that most of them have come from China."

But the topic will undoubtedly be addressed again when Trump meets with Chinese president Xi Jinping on November 8 and 9.

China has long been alleged as the primary source of fentanyl in the United States; a February 2017 report from the Congressional U.S.-China Economic and Security Review Commission traced the synthetic opioid and its analogues back to China's chemical and pharmaceutical industries, said to manufacture the drug with "little regulatory insight."

But at the press conference on November 3, which was attended by American and Chinese officials, Wei indicated that the evidence linking his country with fentanyl in the West was "limited," and pointed to Mexico—which law enforcement officials have suggested is the entry point for fentanyl into the United States via drug cartels—as remaining silent on the issue of Chinese-made fentanyl coming into their country.

Wei further added that his country has banned more than 20 synthetic opioids, including carfentanil and three fentanyl analogues on March 1—moves that the Drug Enforcement Administration has credited with drops in seizures of such drugs in the States. "We did this even when there is no widespread fentanyl abuse in China," said Wei. "We were aware of the crisis in the U.S. and took the U.S. concern into consideration."

Wei also said that his country regretted the indictment of two Chinese nationals for the manufacture and sale of fentanyl and other synthetic drugs to the United States in October of this year. "The suspects' arrest will depend on evidence provided by the U.S. and evidence discovered by the Chinese police," he stated. "The two countries have different laws governing such substances—China can only take actions based on its domestic laws and what its law enforcement officers find."

Lance Ho, the DEA's country attaché in Beijing, was present at the conference and suggested that continued vigilance by China to regulate the production of synthetic drugs will have a positive impact on the U.S. opioid crisis.

"Once China controls the substances, it has dramatic effects in the United States in terms of lives saved," he said. "Once they do that, we see a decrease in the usage in the United States."

Ho added that collaboration between the two countries remains key to prevention efforts. "What's really important is the real-time information sharing—that's vital," he said.