"Taking into account the fact that both vessels remained near each other with their lights switched off, there is a possibility that both tankers were involved in some activity. The Japanese government, guided by a thorough analysis, assumes that they might have transferred goods from one ship to another, which is prohibited by the UNSC resolutions," the statement said.
According to the statement, the same information has also been transmitted to the United States, which had previously requested the Japanese Self-Defense Forces' assistance in monitoring North Korean vessels.
The UN Security Council has introduced several rounds of sanctions, in response to the North's nuclear and missile tests, with the latest unanimously adopted last December. The sanctions limited North Korea's oil dealings to 4 billion barrels per year and made oil exporters working with the country report on their deliveries to the UN Security Council.
Pyongyang rejects the new sanctions, saying that the country's nuclear weapons are a self-defense deterrence against nuclear threats and blackmail by the US.
Meanwhile, the US revealed last week its decision to impose restrictions on 27 shipping and trading companies, 28 vessels and 1 individual from several countries, including North Korea.
The US Treasury Department also released a global shipping advisory with information about North Korea's deceptive shipping practices and warned about potential sanctions against those who enable the shipment of goods to or from North Korea.