WASHINGTON—The U.S. is convening a multinational coalition to significantly expand surveillance of ships smuggling fuel to North Korea in violation of United Nations sanctions, American military officials said.

The coalition is the first unified international effort to monitor the ship traffic in the year since the Trump administration launched its “maximum-pressure” sanctions campaign, aimed at strong-arming North Korea into abandoning its nuclear and missile programs. Surveillance efforts until now have been a hodgepodge...