The fentanyl trade has long been a thorn in the side of relations between China and the United States, with Washington accusing Beijing of turning a blind eye, something it denies.

China added two previously unclassified precursors -- 4-piperidone and 1-boc-4-piperidone -- to a list of substances subjected to stricter regulation from July 20, a statement said on Monday.

"This move is an independent initiative by the Chinese government to fulfil its obligations as a signatory to the UN drug control conventions," foreign ministry spokesman Guo Jiakun told a regular briefing when asked if the move was done in cooperation with the United States.

"It aims to strictly regulate precursor chemicals and reflects... its role as a responsible major country," he added.

The US and China this month agreed to a temporary truce in a standoff that saw tariffs hiked to eye-watering levels and upended global supply chains.

In May, Washington and Beijing agreed to drastically reduce these tit-for-tat tariffs for 90 days, but the US additional tariff rate remained higher than China's because it includes a 20 percent levy over President Donald Trump's complaints about Chinese exports of chemicals used to make fentanyl.

The US has seen tens of thousands of deaths related to opioids like fentanyl annually, though the latest figures, for 2023, showed a decrease from the prior year.

"The US has ignored China's goodwill and unreasonably imposed on China fentanyl tariffs," Guo said.

"If the US truly wants to cooperate with China, it should face up to objective facts, correct its mistakes and engage in dialogue with China on the basis of equality, respect and mutual benefit," he added.