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

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

The adviser 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.

Last month, the US State Department has described talks with North Korea as "good discussions", disputing claims by the top North Korean envoy that the meeting broke down.

North Korea blamed the US for bringing "nothing to the negotiation table" at the meeting, which came after months of stalemate