Japanese military sources told the Kyodo news agency that warships have been monitoring the Sea of Japan and the Yellow Sea east and west of the Korean Peninsula at the request of the US since last month.
The Japanese ships are alerted to any suspicious sightings by military planes, the outlet said, and pass photos of them on to the United States. They are not allowed to search the vessels.
The United States and 16 other nations on Friday pledged to intercept ships heading to North Korea and called on all UN member states to abide by UN’s December sanctions that cut the amount of refined fuel the North can import and halted ship-to-ship fuel transfers.
Earlier, South Korean authorities intercepted the ship under the flag of Panama on suspicion of carrying and selling oil to Pyongyang. The event comes days after Seoul announced it had intercepted and searched the Hong Kong-flagged ship Lighthouse Winmore, which had delivered 600 tons of processed oil to a North Korean ship.