Many of the vessels bore crude disguises, from fictitious names painted on their bows to fake hatches built of canvas and wood to give them the look of a cargo ship. Once at sea, they would rendezvous at night with a foreign tanker, toss hoses over the rail and fill their hulls with illicit oil before sailing home again.

Such fuel-smuggling runs are not uncommon in the Yellow Sea, yet the scale of activity over the spring and summer startled U.S. and East Asian intelligence officials who tracked the ships’ movements by satellite. By late August, spy agencies had counted 148 of these secret maritime transfers — for a total of between 800,000 and 1.4 million barrels of oil, gasoline and diesel — with the volume increasing in recent months as diplomacy with North Korea picked up steam.

A confidential U.N. report last month identified 40 vessels and 130 companies, many with Chinese or Russian ties, as contributing to a “massive” spike in smuggling that analysts say has helped stabilize North Korea’s economy just as U.S. diplomats are attempting to compel leader Kim Jong Un to give up his nuclear arsenal.