The United States is expressing concern about the cargo that a North Korean ship was carrying when it returned home from international waters in late May after failing to find a foreign port to dock.

State Department Deputy Spokesman Mark Toner told reporters on Monday that the vessel was suspected of carrying materials prohibited by United Nations Security Council resolutions.

Toner said the United States requested that the ship permit a cargo inspection, but the ship’s master denied access for a consensual boarding and inspection. Toner said the vessel later changed course at sea and is believed to have returned to North Korea.

The spokesman added that he could not get into any specifics about the cargo, due to operational considerations. However, he did say that the ship master's refusal to let the U.S. board and the ship's decision to return North have raised suspicions.