Japanese military spotted what appeared to be a sanctions violation by North Korea Tuesday night.

A Japanese Maritime Self-Defense Force plane and ship discovered a North Korean tanker “lying alongside a small vessel of unknown nationality … on the high sea,” off the coast of China.

The discovery was then reported to the United Nations.

“Following a comprehensive assessment, the government of Japan strongly suspects that they conducted [a banned] ship-to-ship transfer,” said Japan’s Ministry of Foreign Affairs in a statement.

The United States and U.N. have imposed strict economic sanctions on North Korea following numerous ballistic and nuclear weapons tests. The latest and strictest set of sanctions were passed last year after North Korea test-fired an intercontinental ballistic missile that showed the country could theoretically strike anywhere in the U.S. The sanctions included prohibiting the trade of oil to North Korea.

North Korea employs smuggling and black market purchases in order to fund and supply itself. One way its been reported that the country had skirted sanctions is through ship to ship transfers. North Korean oil tankers will meet a ship that has agreed to illicitly supply the country oil in the middle of the ocean. The North Korea vessel will then take the oil back to its home port.

There have been several instances of countries catching North Korean ships allegedly violating sanctions at sea in the past several months.

The sanctions are meant to cripple North Korea’s already cash-strapped economy and force its leader Kim Jong Un to give up his nuclear ambitions, something North Korean media has claimed he would never do.

The sanctions coupled with South Korea’s own restrictions on North Korea, especially around traveling between the two countries, has complicated their participation in the Winter Olympics in Pyeongchang. Given North Korea’s poverty, and in the spirit of good diplomacy, South Korea had agreed to foot the country’s Olympic bill, however.