South Korea's nuclear envoy Lee Do Hoon told a senior U.S. diplomat on Tuesday that the "coming several weeks and months will be critical" in terms of laying the groundwork for the denuclearization of North Korea.

The meeting in Seoul between Lee and Susan Thornton, acting U.S. assistant secretary of state for East Asian and Pacific affairs, came just days ahead of a planned summit between South and North Korea, to be followed by the first-ever U.S.-North Korea summit talks by early June.

At the outset of the meeting, Lee and Thornton both welcomed the news that U.S. President Donald Trump's pick for secretary of state, Mike Pompeo, was approved by a U.S. Senate committee, setting the stage for him to assume the post.

Pompeo, currently director of the Central Intelligence Agency, is expected to play a key role in negotiations with North Korea over the country's denuclearization. As Trump's special envoy, he recently met North Korean leader Kim Jong Un in Pyongyang.

Calling the senate approval "excellent news," Lee, special representative for Korean Peninsula peace and security affairs, told Thornton, "I'll hope that we will be able to have the consultations as closely as possible at different levels."

Thornton agreed on the importance of close coordination and consultation between the two countries and said, "We'll be keeping in close touch and watching carefully" as events unfold.

(Lee Do Hoon, left, and Susan Thornton)

The U.S. diplomat, who has been in South Korea from Sunday, met South Korean Foreign Minister Kang Kyung Wha on Monday and agreed that it is important for Seoul and Washington to show close coordination before the two planned summits with North Korea, according to the South Korean Foreign Ministry.

North Korea's announcement last Saturday to suspend nuclear tests and intercontinental ballistic missile launches, as well as dismantle its only known nuclear test site, has been welcomed by Trump as "big progress."

But with no direct pledge to give up its nuclear arsenal, doubts linger in the White House over whether Kim is serious about North Korea's earlier reported commitment to denuclearization, or is just trying to win concessions, such as the easing of economic sanctions imposed on his country over its weapons programs.

South Korean President Moon Jae In has reacted positively to North Korea's latest decision, saying it is "a sincere step taken" ahead of the two summits.