President Donald Trump and North Korea's leader Kim Jong-un stand on North Korean soil while walking to South Korea in the Demilitarized Zone (DMZ) on June 30, 2019, in Panmunjom, Korea.

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

Moon said U.S. 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.

"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 directly received a letter from Trump and ridiculed South Korea for trying to "meddle" in U.S.-North Korea relations.