WASHINGTON -- South Korea and the United States are united in keeping sanctions on North Korea until the regime makes "substantial" progress toward denuclearization, a top Seoul diplomat said Monday.

First Vice Foreign Minister Lim Sung-nam was speaking at a forum assessing last week's historic summit between US President Donald Trump and North Korean leader Kim Jong-un, which produced an agreement to pursue complete denuclearization of the Korean Peninsula in return for security guarantees for the regime.

In follow-up negotiations, North Korea is expected to demand an easing of sanctions that were tightened last year in response to its nuclear weapons and ballistic missile tests.

"The sanctions relief will be possible only when we see substantial progress in the area of denuclearization," Lim said at the forum jointly organized by the Center for Strategic and International Studies and the Korea Foundation. "That position has been also clearly stated by (US) Secretary (of State Mike) Pompeo as well as President Trump, so there's no daylight whatsoever as of now with regard to the need to see the substantial progress in the area of denuclearization before we make any kind of specific moves in terms of relieving the sanctions."

Until that condition is met, he said, South Korea will be "prudent at all times" and remain committed to "full implementation" of the sanctions regime.

Lim is the first high-ranking South Korean official to visit Washington since Tuesday's Trump-Kim meeting in Singapore.

In the afternoon he met with Deputy Secretary of State John Sullivan to discuss the allies' joint approach to future negotiations between Washington and Pyongyang on implementing the Singapore statement, according to Seoul's foreign ministry.

The officials pledged close coordination between their governments at all levels.

Lim plans to have a dinner meeting with Susan Thornton, the acting top US diplomat for East Asia, and other American officials. He is scheduled to meet separately with US lawmakers to continue discussions on implementing both the Singapore statement and the Panmunjom Declaration.

The Panmunjom Declaration was adopted after an historic inter-Korean summit on April 27 and also calls for complete denuclearization of the Korean Peninsula.

Lim will head to Boston on Tuesday to take part in a seminar on regional security. (Yonhap)