U.S. Defense Secretary Jim Mattis said there is progress on dismantling North Korea's nuclear weapons program, although it remains a difficult issue.

Mattis told reporters traveling with him to Vietnam Monday that the efforts to denuclearize North Korea continue to be diplomatically-led.

He cited U.S. Secretary of State Mike Pompeo's recent trip to Pyongyang, where the top American diplomat met with North Korean leader Kim Jong-un to push progress on denuclearization and plan a second summit between Kim and U.S. President Donald Trump.

"There is progress," Mattis said. "We know it's going to be difficult as they deal with this difficult issue. No surprise there."

Trump and Kim agreed at their first summit in June to work toward "complete" denuclearization of the Korean Peninsula in exchange for security guarantees from the U.S.

Critics have cast doubt over the deal, saying the regime has yet to take serious dismantlement steps. They accuse Trump of giving major concessions to the North, such as a suspension of joint exercises between the U.S. and South Korean militaries.

Mattis said the allies continue to train their troops and collaborate military-to-military.

He also said the international sanctions regime against the North remains in place.

"The sanctions patrols to enforce the unanimous U.N. Security Council resolution sanctions, specifically on ship-to-ship transfer, are being maintained," he said. "On a steady basis we have a number of countries helping on that. I think it's up to five countries right now. I may be wrong, but it's around that number. And the information-sharing among those countries is going well."

Source: Yonhap News Agency