U .S. and Chinese officials may be able to settle many of their differences to reach a trade deal and avert punishing tariffs, but they remain far apart on one issue: the U.S. demand that China stop purchasing oil from Iran and Russia.

Under the Joint Comprehensive Plan of Action, the Iran nuclear deal agreed in 2015, sanctions can be reimposed on Iran's military and trade sectors—targeting key figures with travel bans and assets freezes—if Tehran is found to have violated the JCPOA. Tehran says the mechanism lacks legal standing.

Beijing insists it will continue to secure its energy supplies based on national interests despite pressure from Washington and the threat of punitive tariffs.