U.S. President Donald Trump said Monday that a fourth summit with North Korean leader Kim Jong Un “could happen soon” as diplomacy over the North’s nuclear program has stalled.

Trump didn’t elaborate in comments to reporters at the United Nations in New York, and it was not clear if concrete plans were in the works for another summit meant to address a growing diplomatic impasse over the North’s development of nuclear-armed missiles targeting the U.S. mainland.

There is extreme interest, especially in Japan and South Korea, in whether Trump and Kim can settle differences that have led to the stalemate despite past high-profile meetings. The sides have failed even to set up lower-level negotiations on restarting talks.

At the heart of the dispute is the impoverished North’s desire for relief from harsh sanctions imposed as it has boosted its nuclear and missile capabilities. Washington, however, is demanding that Pyongyang first take more comprehensive steps to dismantle a nuclear program that has been painstakingly built over decades.

In recent months, the North has displayed its unhappiness with the tough U.S. stance in a series of short-range weapons tests that experts see as an attempt to increase pressure on Washington and Seoul and build leverage ahead of any new talks.

Trump has repeatedly played down tests of weapons that could strike allies South Korea and Japan, which host tens of thousands of U.S. troops. He also has trumpeted the strength of his relationship with Kim.

As he arrived at the U.N. for the start of the annual General Assembly of world leaders, Trump responded to a question about when he planned to meet with Kim, saying, “It could happen soon. It could happen soon.”

Trump was scheduled to meet later Monday with South Korean President Moon Jae-in, the driving force behind the initial diplomacy that led to the first Trump-Kim summit last year and replaced threats of war in 2017.

North Korea reportedly hasn’t sent its top diplomat to the U.N. meetings, making the Moon-Trump talks potentially the highest-level diplomatic opportunity to make progress this week on the nuclear standoff.

Speaking Monday of that first summit with Kim, in Singapore, which critics have called more surface than substance, Trump said, “People don’t give that meeting as much credit as it deserves and the relationship we established. It was a tremendous success.”

Trump is set to speak before the U.N. General Assembly on Tuesday, two years after he used that podium to deride Kim as “Little Rocket Man” and to threaten to destroy North Korea.