SEOUL/WASHINGTON (Reuters) - South Korea’s national security adviser is in Washington to meet his U.S. counterpart, John Bolton, ahead of an expected summit between U.S. President Donald Trump and North Korean leader Kim Jong Un, officials in Washington and Seoul said.

FILE PHOTO: Republic of Korea's National Security Advisor Chung Eui-Yong attends a meeting with Chinese President Xi Jinping and South Korean Ambassador to China Noh Young-min at the Great Hall of The People in Beijing, China March 12, 2018. REUTERS/Etienne Oliveau/Pool

The U.S. National Security Council asked Chung Eui-yong to fly to the United States to discuss matters related to the summit, a South Korean presidential official told reporters on Friday.

The United States had asked that the visit be kept quiet due to the issues to be addressed at meetings there, said the official, who declined to be identified.

A senior administration official in Washington confirmed Chung’s visit and his meeting with Bolton.

South Korea has been working closely with old ally the United States on efforts to rein in North Korea’s nuclear and missile programs, with an easing of tensions in recent months after threats of war from North Korea and Trump.

South Korean President Moon Jae-in and Kim pledged to work for the “complete denuclearisation” of the Korean peninsula at a summit on their heavily fortified border on April 27.

Moon also spoke by telephone with Chinese President Xi Jinping, according to South Korea’s presidential office.

Moon stressed that China’s support was crucial for denuclearisation and to establish permanent peace, the office said. China is North Korea’s lone major ally.

Moon and Xi agreed to cooperate in the process of signing a peace treaty to end hostilities on the peninsula, it added.

“China is ready to join with the international community, including North and South Korea, to play its due, active role in pushing forward the process for a political solution to the peninsula issue to finally achieve lasting peace in the region,” Chinese state media cited Xi as saying.

Impoverished North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a mere truce, not a treaty.

North Korea says it is ready to discuss denuclearisation and give up its nuclear program as long as the security of Kim’s regime is guaranteed.

Among issues to be decided before Trump can meet Kim are where and when they will hold their summit.

Trump has suggested holding the meeting, which is expected in late May or early June, at the border village of Panmunjom in the demilitarized zone between the two Koreas, where Moon and Kim met.

This would require close coordination with Seoul, although officials in the South Korean president’s office have said there has been no official requests to prepare the venue.

Chung may address the issue of a venue in Washington but he was more likely to discuss a “bigger deal” with U.S. officials pertaining to North Korea, the South Korean official said.

Chung was last in Washington shortly before the Moon-Kim summit.

Trump has ordered the Pentagon to prepare options for reducing the number of U.S. troops in South Korea, the New York Times reported, citing several people briefed on the deliberations.

The United States stations 28,500 troops in South Korea, a legacy of the Korean War.