A U.S. special envoy said on Friday the United States was ready to hold constructive talks with North Korea to follow through on a denuclearisation agreement reached by the two countries last year, South Korea's foreign ministry said.

Stephen Biegun told his South Korean counterpart, Lee Do-hoon, that Washington wanted to make "simultaneous, parallel" progress on the agreement reached at a summit between President Donald Trump and North Korean leader Kim Jong Un in Singapore last year, the ministry said in a statement.

Both sides agreed to establish new relations and work towards denuclearisation of the Korean peninsula.

But negotiations have stalled since a second summit in Vietnam in February collapsed as the two sides failed to narrow differences between U.S. calls for denuclearisation and North Korean demands for sanctions relief.

Russian President Vladimir Putin told South Korean President Moon Jae-in at a summit on the sidelines of G20 meeting in Japan that Kim had told him in April security guarantees were key, and that corresponding measures were needed to realize denuclearisation, according to South Korea's presidential office.

The envoys' meeting, and Putin's remarks, took place ahead of Trump's visit to South Korea this weekend, during which Trump and Moon are expected to discuss ways to kick-start dialog with the North.

Reclusive North Korea has pursued nuclear and missile programs for years in defiance of U.N. Security Council and U.S. sanctions.

"Biegun said the upcoming summit would provide a crucial chance to foster peace and prosperity of the Korean peninsula," the ministry said.

Lee offered positive views on a recent cordial exchange of letters between Trump and Kim and on Chinese President Xi Jinping's summit with Kim early this year.

U.S. Secretary of State Mike Pompeo raised hopes for a revival of talks after the letters, while Xi told Moon on Thursday that Kim's resolve for denuclearisation and dialog remained unchanged.

Biegun also met South Korean Unification Minister Kim Yeon-chul in charge of inter-Korean affairs, during which they vowed utmost efforts to revive nuclear talks, the foreign ministry said.

Moon has said Washington is in behind-the-scenes talks with Pyongyang over a possible third summit and has proposed a fresh round of talks. But the North said on Thursday the United States had become "more and more desperate in its hostile acts" even as it spoke of dialog.

North Korea's state media KCNA on Friday urged the United States and South Korea to scrap their plans to carry out joint military drills in the summer.

The allies have replaced their annual major exercises with smaller-scale programs following last year's detente. But Pyongyang sees the drills as a rehearsal for war and demands they be called off.

"This amounts to a wanton challenge to the desire and expectation of all the Koreans and the international community for peace and stability on the Korean peninsula and an act to create the atmosphere of confrontation and danger of war again," KCNA said in a commentary.