Washington has long accused Beijing of turning a blind eye to the deadly fentanyl trade, which US authorities estimate caused over 70,000 overdose deaths last year, charges China denies.

Here AFP looks at where the issue currently stands:

What is fentanyl and where does it come from?

The United States is facing an epidemic of deaths caused by fentanyl, a synthetic opioid 50 times more powerful than heroin and much easier and cheaper to produce.

It is now the leading cause of death among people aged 18 to 45, US officials have said.

The US Drug Enforcement Agency has said China is "the main source for all fentanyl-related substances trafficked into the United States".

And while the Congressional Research Service acknowledged this year that direct supplies of the drug from China had been stemmed by stricter controls from Beijing in 2019, it said the move had simply shifted the supply lines.

Instead of the drugs being supplied directly via international courier services, it said, chemical components are instead shipped from China to Mexico, where they are then made into fentanyl and smuggled across the border.

Many of those components are legal in China and have legitimate medical use as painkillers, making prosecution tricky.

Beijing, which insisted there is "no such thing as illegal trafficking of fentanyl between China and Mexico", has promised to crack down.

In response to Trump's threat this week to impose a 10 percent tariff, its foreign ministry said that "China is one of the world's toughest countries" when it comes to drugs.

What has the US done to crack down?

President Joe Biden's administration has made the fight against fentanyl a priority.

Last October, it slapped sanctions on over two dozen China-based entities and individuals alleged to be the "source of supply" for many US-based narcotics traffickers, dark web vendors, virtual currency money launderers and Mexican cartels.

The group, which included a Wuhan-based company and a number of other firms based in Hong Kong and the mainland, was alleged to be responsible for the shipment of approximately 900 kilograms of "seized fentanyl and methamphetamine precursors" shipped to the United States and Mexico.

"The global fentanyl supply chain, which ends with the deaths of Americans, often starts with chemical companies in China," US Attorney General Merrick Garland said.

China condemned the moves at the time as part of a US campaign of "pressure and sanctions" against it.

What have the US and China agreed to?

China-US talks on drug control had stalled in the face of some of their worst relations in years.

But following a summit between Biden and Chinese President Xi Jinping in San Francisco last November, Washington and Beijing agreed to restart talks.

This summer, a counternarcotics working group convened in Washington and China announced it would step up its regulation of three key fentanyl precursors.

But it remains to be seen whether the latest curbs will fully stop the cross-border traffickers -- who the Justice Department said "adapt to tightening restrictions".

Experts say that manufacturers are able to develop new variants of the synthetic precursors faster than they can be identified and added to scheduled lists of substances controlled by Chinese authorities.

Vanda Felbab-Brown, an expert on organised crime at the Brookings Institution, said Beijing needs to take a tougher line against domestic firms involved in the trade.

"We are nowhere close to robust indictments, robust prosecutions in either the money laundering sector or smuggling of precursors to the Mexican cartels," she said in a recent podcast.

Will the tariffs work?

Trump looks poised to take a hard line against China once in office, especially after his recent selection of several Beijing critics to fill key posts in his administration.

But whether threatening tariffs will spur greater action from Beijing is unclear.

China's foreign ministry, responding to Trump's announcement of tariffs, said it "remains ready to continue counternarcotics cooperation with the US".

But, it added, Beijing hopes Washington "will not take China's goodwill for granted".

Compounding the problem are expansive money laundering networks that underpin the trade, which experts say requires closer coordination between Washington and Beijing to curb.

"International drug cartels are increasingly turning to specialized Chinese criminal gangs for swift, cheap and secure money laundering services," wrote Zongyuan Zoe Liu, in a September report for the Council on Foreign Relations.

"Obtaining support from Beijing to stop the flow of illicit fentanyl and its precursor chemicals is an important first step in addressing the supply problem," Liu wrote.