U.S. President Donald Trump and South Korean President Moon Jae-in on Sunday discussed threats by North Korea’s leader Kim Jong-Un to cancel its summit with Trump.

Trump and Moon, in a phone conversation on Sunday, “exchanged views on various actions taken by North Korea recently”, Moon’s office said in a statement.

The two leaders agreed to “work closely” for the success of the June 12 summit in Singapore, which would be the first meeting between a sitting U.S. President and a North Korean leader.

Both Trump and Moon are due to meet in Washington, DC on Tuesday.

North Korea abruptly threatened to pull out of the planned summit next month because of U.S. demands for “unilateral nuclear abandonment”, the North’s official KCNA news agency said.

North Korea also cancelled at the last minute a high-level meeting with the South, protesting joint military drills between South and the U.S.

North Korea’s sudden change in attitude followed a weeks-long charm offensive that has seen Kim hold a historic summit with Moon and met twice with Chinese President Xi Jinping.

At a dramatic summit last month in the Demilitarised Zone dividing their two countries, Kim and Moon pledged to pursue nuclear disarmament and a peace treaty.

Pyongyang also raised hopes ahead of the U.S. summit by announcing it would destroy its nuclear testing site next week