A commission set up by the U.S. Congress said Chinese authorities are not doing enough to halt the flow of the synthetic opioid fentanyl and related chemicals into the U.S., where the substance has been linked to an epidemic of overdose deaths.

“U.S. officials have proposed strategies for Beijing to systematically control all fentanyl substances, but the changes have not been approved by the Chinese government,” Sean O’Connor, policy analyst for the U.S.-China Economic and Security Review Commission, wrote in a Nov. 26 report.

Fentanyl exporters have skirted Chinese laws by shifting to analogues, or molecules that have similar effects on the body, but do not fall under bans the country has imposed on fentanyl itself, according to the U.S. commission. China has been too slow to add new categories of analogues to the list of prohibited substances, the U.S. report said.

A spokesman for China’s foreign ministry said the government attaches great importance to the issue and has been taking effective measures to deal with it. China has listed as controlled 25 fentanyl substances and five precursors, shared information with other countries and enhanced domestic law enforcement, Geng Shuang, foreign ministry spokesman, said Tuesday during a daily briefing through a translator.

President Donald Trump in October 2017 declared widespread opioid abuse a public health emergency and vowed to use the federal government’s legal powers to pursue companies that helped fuel the epidemic. Trump said at the time that he would raise the issue of Chinese fentanyl making its way to the U.S. with Chinese President Xi Jinping as a top priority “and he will do something about it.”

The commission reinforced a conclusion made in a February 2017 brief calling China “the largest source of illicit fentanyl and fentanyl-like substances in the United States.” Domestically, China does not have a fentanyl abuse problem, according to the report.

“The U.S. has been accusing China of being the source of their fentanyl, but so far it’s failed to offer any solid evidence to us and the leads they gave us are very limited,” said Geng. The U.S. could do more in reducing demand for the drug and most new fentanyl substances are manufactured from labs in the developed world such as the U.S. and Europe, he said.

The opioid epidemic has emerged as one of America’s most pressing public health matters, claiming a life every 19 minutes, according to the U.S. Surgeon General. Cost estimates range, but a 2016 study in the Medical Care Journal estimated the annual economic cost of opioid overdose, abuse and dependence at $78.5bn.

The U.S.-China Economic and Security Review Commission was created by Congress in October 2000 to investigate and submit an annual report on the national security implications of trade between the U.S. and China.