TOKYO—Japan said it spotted a North Korean oil tanker appearing to take on cargo in international waters in the East China Sea in a possible violation of United Nations sanctions, a rare case of Tokyo making public information about its monitoring of North Korean trade.

In the early hours of Jan. 20, a Japanese military plane observed the North Korean tanker Rye Song Gang 1 alongside the Dominican-registered tanker Yuk Tung in the East China Sea, Japan’s foreign ministry said in a statement Wednesday.

...