A Japanese patrol plane spotted contact between tankers of North Korea and Commonwealth of Dominica in the East China Sea, raising suspicion of a possible transfer of cargo in violation of U.N. sanctions on the North, government sources have said.

The Maritime Self-Defense Force P-3C aircraft took photos of the scene near Shanghai on Saturday as it was patrolling for illicit transfer of refined oil at sea by North Korea.

Foreign Minister Taro Kono said Tuesday that the government will take tougher measures on such activity by North Korea.