Updated

Japan says its military has witnessed a ship-to-ship transfer on the high seas that it "strongly suspects" violates U.N. sanctions on North Korea.

The Foreign Ministry said late Tuesday that a surveillance plane and an escort ship saw a North Korean-flagged tanker alongside a smaller ship about 250 kilometers (150 miles) off Shanghai in the East China Sea.

The Feb. 16 sighting was the third one reported by Japan in the past month. The Foreign Ministry said Japan had notified the U.N. Security Council of the transfer.

The U.N. has toughened sanctions on North Korea to pressure it to give up its nuclear weapons development. Japan and the U.S. have said the sanctions must be enforced strictly to be effective.