Beijing criticised US authorities on Saturday for charging four Chinese companies and eight individuals with trafficking chemicals used in manufacturing fentanyl, calling the arrests "arbitrary detention".
The US Justice Department is intensifying a crackdown on the synthetic opioid responsible for hundreds of thousands of overdose deaths in America over the past decade.
Its latest move is the first time Washington has charged Chinese firms for trafficking fentanyl precursor chemicals to the United States, rather than Mexico, the origin of most fentanyl found in America.
Two of the eight individuals were arrested and taken into custody in Honolulu, Hawaii.
"This is a typical example of arbitrary detention and unilateral sanctions," the Chinese foreign ministry said in a statement.
"It is completely illegal and seriously damages the basic human rights of Chinese citizens and Chinese companies. China strongly condemns this."
The four companies are Hubei Amarvel Biotech, Anhui Rencheng Technology, Anhui Moker New Material Technology and Hefei GSK Trade.
They have been charged in three separate cases filed in federal court in New York.
Among the eight people indicted are executives and employees of the four firms.
The pair who were arrested are employees of Hubei Amarvel Biotech and were detained in Hawaii after being expelled from Fiji on June 8.
Their detention came just days after US Secretary of State Antony Blinken visited the Chinese capital, where he met with President Xi Jinping.
Fentanyl has become an important issue in the tense relationship between Washington and Beijing, with Blinken raising it during his visit.
The charges "seriously undermine the foundation of China-US anti-drug cooperation", the Chinese foreign ministry said.
"China urges the United States to... release illegally detained Chinese citizens."
Fentanyl is a synthetic opioid that is 50 times more powerful than heroin and much easier and cheaper to produce.
Beijing listed all fentanyl-related substances as controlled narcotics in 2019, regulating their production and sale.
Although Mexico has been the main source of fentanyl sold in the United States, Washington has increasingly focused its attention on China-based suppliers of ingredients sent to Mexico.
