Foreign Minister Taro Kono said Saturday that U.N.-prohibited ship-to-ship transfers of petroleum products to North Korea have allowed for an "extremely big loophole" in enforcing U.N. sanctions on Pyongyang.

Kono made the remark to reporters in Los Angeles, a day after he agreed with U.S. Secretary of State Mike Pompeo during telephone talks to strengthen coordination with other countries in blocking such illegal activity.

Kono declined to reveal details about what Pompeo said regarding the cancellation of his planned trip to North Korea. The Japanese minister only said that the cancellation came with "an extremely reasonable reason."

On Friday, U.S. President Donald Trump nixed Pompeo's planned trip to North Korea, saying the country is "not making sufficient progress" on its denuclearization despite leader Kim Jong Un committing to "complete" denuclearization during a historic summit with Trump in June in Singapore.

In an effort to sever financial flows to Pyongyang's weapons of mass destruction programs in accordance with U.N. Security Council sanctions resolutions, the United States have sanctioned Chinese and Russian entities for their involvement in illicit shipments to North Korea.

Kono was on the last leg of his U.S. tour that had taken him to Honolulu and San Francisco.