SEOUL (Reuters) - 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 U.S. flexibility approaches.

Residents hold US and North Korean flags while they wait for motorcade of North Korea's leader Kim Jong Un en route to the Metropole Hotel for the second US- North Korea summit in Hanoi, Vietnam February 28, 2019. REUTERS/Kham/File Photo

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.

North Korean leader Kim Jong Un in April gave the United States a year-end deadline to show more flexibility in their denuclearisation talks, 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.

U.S.-North Korea nuclear negotiations have been deadlocked with working-level talks in October ending fruitlessly.

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, will the third North Korea-United States summit be possible,” Chung told a news conference to mark the half-way point of President Moon Jae-in’s five-year term.

“As you know, the North side has shown the year-end deadline, considering that position of the North Korean side, we are closely coordinating with the U.S. side.”

As the talks between the United States and North Korea have stalled, so have efforts to improve ties between the two Koreas, despite efforts by the South Koreans to nudge them forward.

In the latest sign of strained ties on the peninsula, the North Korea leader Kim ordered the removal of “shabby”, “capitalist” facilities the South built in the North’s Mount Kumgang resort.

North Korea has also rejected a South Korean offer of talks, proposing that communication be limited to exchanges of documents.

South Korean President Moon also faces diplomatic tension with Japan.

Relations between the important U.S. allies have sunk to their worst state in decades after South Korea’s top court ordered Japanese firms to compensate wartime forced labourers.

Japan imposed export curbs on high-tech materials bound for South Korea.

In response, South Korea decided not to renew an intelligence-sharing pact with Japan, due to expire later this month.

Chung said South Korea would be willing to reconsider its decision to walk away from the pact if ties with Japan were normalised. But he blamed Japan for the worsening relations.