US broadcaster VOA reported that a Belize-flagged ship docked at South Korea's southeastern port city of Pohang over the weekend after being spotted at the Russian port of Nakhodka with black material believed to be North Korean coal.
UN Security Council Resolution 2371 bans North Korean exports of coal and other mineral resources that could generate revenue for the regime's development of nuclear weapons and ballistic missiles.
UN member states are also required to stop and inspect vessels suspected of engaging in illicit activities with the North.
"The Republic of Korea is a faithful and reliable partner in the maritime implementation of UN Security Council Resolutions," a State Department spokesperson said in response to a Yonhap query.
"The United States and the ROK work closely together on North Korea issues, and remain in close contact to coordinate our unified response to the DPRK."
DPRK is an acronym for North Korea's formal name, the Democratic People's Republic of Korea.
"We are aware that the ROK government has initiated an investigation," the spokesperson added.
South Korea's foreign ministry said Tuesday that an inspection of the ship determined the coal to be Russian. It also said there was "no confirmation of a violation of" the U.N. Security Council resolution.
South Korea has also been looking into separate allegations that several foreign cargo ships loaded with North Korean coal docked at South Korean ports in violation of international sanctions against Pyongyang. (Yonhap)