WASHINGTON • In his series of tweets announcing new tariffs on China, US President Donald Trump said Chinese President Xi Jinping promised to stop the sale of fentanyl to the US, but "this never happened, and many Americans continue to die!"
Fentanyl, a chemical many times more powerful than heroin, caused nearly 32,000 overdose deaths in the US last year.
In 2014, it was detected in fewer than 6,000 fatal overdoses.
In April this year, China, responding to Mr Trump's concerns raised in meetings with Mr Xi, announced it would ban all variants of fentanyl, raising hopes that it could slow the flow of the powerful chemical.
But on Thursday, hours after Mr Trump complained, Mr Jorgan Andrews, Deputy Assistant Secretary of the Bureau of International Narcotics and Law Enforcement Affairs, told journalists: "We have very little information thus far indicating that the government of China is following through on its May 1 decision to control fentanyl and all of the fentanyl variants in the class."
The US will continue to press China to follow through on its promise, he added.
"The emergence of dangerous illicit synthetic opioids such as fentanyl has… transformed America's opioid crisis into a deadly phenomenon with complex transnational and criminal linkages," Mr Andrews said.
Illicit manufacturers get around bans on fentanyl by altering formulas quickly, creating new variants that do not fall under the bans, he noted. China's new controls would make it illegal to do that "if they are properly enforced".
"The production and distribution of fentanyl occurs in small quantities," Mr Andrews said. "Therefore, it will require strong Chinese enforcement and cooperation with the United States and other global partners to detect and interdict those smaller shipments."
The profitability of synthetic drugs, including fentanyl, was such that it was hard to imagine any country which would not be affected, he warned.
"Illicit synthetic drugs pose the most significant global drug threat of the next decade, in our view," he said.
"The most worrisome thing is that they are so potent and so lethal that they get cut with other substances and other drugs, and users don't realise what they are taking, and they can't control the dosage and it is easy to die from overdose."