South Korean President Moon Jae-in said on Tuesday it was too early to be pessimistic about stalled denuclearisation talks between the United States and North Korea, saying that Pyongyang had not yet shut the door on negotiations.

Moon said US President Donald Trump's recent letter to North Korean leader Kim Jong Un was a good sign that underscores his commitment to negotiations. Moon was speaking at a news conference at the presidential Blue House.

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"Some were concerned about a new round of provocations just in time for Chairman Kim's birthday," Moon said. "Instead, President Trump sent him birthday wishes to stress his willingness to talk. It was a great idea."

On Friday, a South Korean official said Trump had asked the South Koreans to pass on birthday greetings to North Korea.

Over the weekend, however, North Korea released a statement saying it had already received a letter direct from Trump and ridiculed South Korea for trying to "meddle" in US-North Korea relations.

In the statement, a North Korean foreign ministry official said that although Kim liked Trump on a personal level, he would not make policy based on his personal feelings.

"North Korea made clear that the door to dialogue is not closed by saying they will come back to talks only when their demands are accepted," Moon said of that North Korean response.

"The US-North Korea talks are not active right now, but I would say both leaders - President Trump and Chairman Kim - continue to trust one another and continue with their efforts."

South Korea has been increasingly sidelined as denuclearisation talks between the US and North Korea have stalled.

In his New Year's speech on January 7, Moon said there was a "desperate need" for ways to improve ties with North Korea.

Rising tensions and international sanctions have blocked many of Moon's proposals for inter-Korean projects, and Pyongyang has spent the past year criticising Seoul as being beholden to the US.

On Tuesday, South Korean news agency, Yonhap, reported that Seoul's top nuclear negotiator, Lee Do-hoon, would leave for Washington on Wednesday to meet Deputy Secretary of State Stephen Biegun, who is the chief US negotiator with Pyongyang.

White House national security adviser Robert O'Brien said the US had reached out to North Korea seeking to resume talks, according to an interview published on Sunday by Axios news website.