The United States has expanded its lineup of senior officials handling North Korea affairs, sources said Monday, amid a perceived impasse in negotiations to dismantle the regime's nuclear weapons program.

U.S. Secretary of State Mike Pompeo last month named Stephen Biegun, vice president of Ford Motors, U.S. special representative for North Korea.

Shortly afterward, Marc Knapper, who until recently served as acting U.S. ambassador to South Korea, was named acting deputy assistant secretary of state for Korea and Japan issues, according to diplomatic sources.

To further boost the team, Mark Lambert, the State Department's Korea desk director who doubled as acting deputy assistant secretary for Korea and Japan, as well as acting special representative for North Korea, was assigned to a new role handling North Korea issues at the deputy assistant secretary level.

It's still unclear whether that job will become a new official position within the State Department.

Lambert has extensive experience working on the North Korea issue, including during preparations for the Singapore summit between U.S. President Donald Trump and North Korean leader Kim Jong-un last June.

"It seems highly likely that Lambert will provide working-level support to Special Representative Biegun," one source said, amid reports that the new envoy will soon embark on a trip to South Korea and other countries in the region.

"As the issue of North Korea's denuclearization grows, there is a tendency toward breaking up and strengthening relevant departments," the source added.

Trump and Kim produced a joint statement at the summit that committed North Korea to work toward the "complete denuclearization" of the Korean Peninsula in exchange for security guarantees from the U.S.

Negotiations to flesh out the deal have made little visible progress as each side demands the other hold up its end of the bargain first.

Trump abruptly canceled Pompeo's fourth trip to Pyongyang last month, citing a lack of "sufficient progress with respect to the denuclearization of the Korean Peninsula."

Source: Yonhap News Agency