Experts believe breakthrough in US fentanyl crisis may have started in China

US overdose deaths have plunged, but experts warn the supply shock' from Chinese precursors may only be a temporary fix

As Donald Trump travels to Beijing this week, fentanyl - and China's role in its supply chain - remains an enduring point of acrimony in bilateral relations.

At a UN meeting in March, the US again accused China of failing to stop its chemical industry selling the precursors required to make the potent synthetic opioid, while China suggested the US was shifting the blame for its domestic drug problem.