BEIJING (Reuters) -The U.S. and China launched a joint counter-narcotics working group on Tuesday in the first overt sign of cooperation in tackling the spread of fentanyl since late 2019, before bilateral relations between the superpowers soured. It follows a key summit in San Francisco in November where U.S. President Joe Biden and Chinese leader Xi Jinping agreed to work to curb fentanyl production and export, in a major breakthrough. Fentanyl is a highly addictive synthetic opioid 50 times more potent than heroin. The U.S. has said China is the primary source of the precursor chemicals syn…