The United States is pushing for sanctions on third-country vessels involved in the export of North Korean coal and iron ore, in violation of international law.

A U.S. State Department official told a press briefing in Washington on Monday that the United States had asked the United Nations sanctions committee on North Korea to designate seven ships involved in illegal exports.

The official said that between May and June, the Sierra Leone-flagged Flyfree received coal from North Korean vessels via ship-to-ship transfers in North Korean waters and delivered it to Weifang, China.

The official also cited a separate case in January in which North Korean coal and iron ore were offloaded in China using third-country vessels.

UN procedure states that if no member state objects within five days of the sanctions committee’s circulation of the proposal, it will be automatically adopted.

However, opposition from Russia or China, both permanent members of the UN Security Council, could block the move.

The U.S. action comes as North Korea continues to ignore overtures from U.S. President Donald Trump, who has expressed willingness to meet with North Korean leader Kim Jong-un.