United States 'very actively' asking North Korea to return to talks: South Korea

The United States is "very actively" trying to persuade North Korea to come back to negotiations, South Korea's national security adviser said on Sunday, as a year-end North Korean deadline for US flexibility approaches.

South Korea was taking North Korea's deadline "very seriously", the adviser, Chung Eui-yong, told reporters, at a time when efforts to improve inter-Korean relations have stalled.

Chung made the remarks at a news conference to mark South Korean President Moon Jae-in's halfway point of his five-year term.

North Korean leader Kim Jong Un in April gave the United States a year-end deadline to show more flexibility, and North Korean officials have warned the United States not to ignore that date.

The window of opportunity for progress in dialogue with the United States was getting smaller, a senior North Korean diplomat said on Friday, adding that Pyongyang expects reciprocal steps from Washington by the end of the year.

South Korea has set up "various" contingency plans if the deadline passes without any positive outcome, Chung said, without elaborating.

"Only if talks between high-rank officials happen and lead to substantial progress, the third North Korea-United States summit is possible," he said.

"As you know, the Northside has shown the year-end deadline, considering that position of the North Korean side, we are closely coordinating with the U.S. side for North Korea and the United States to find clues."