U.S. aircraft on Thursday night flew to North Korea to begin retrieving the remains of American troops killed during the Korean war, the White House announced.

American planes retrieved the remains from Wonsan, North Korea, and will return them to Osan Air Base in South Korea. A formal repatriation ceremony will be held on Aug. 1, the Trump administration said.

The White House cast the move as a significant step forward in U.S.-North Korean relations following last month’s historic summit between President Trump and North Korean dictator Kim Jong-un.

“At their historic meeting in Singapore, President Donald J. Trump and Chairman Kim Jong Un took a bold first step to achieve the complete denuclearization of the Korean Peninsula, transform relations between the United States and North Korea, and establish enduring peace,” White House press secretary Sarah Sanders said in a statement. “Today, the Chairman is fulfilling part of the commitment he made to the President to return our fallen American service members. We are encouraged by North Korea’s actions and the momentum for positive change.”

The move was first reported by South Korea-based Yonhap news agency.

It’s unclear how many remains are being transported out of North Korea; the White House said an estimated 5,300 Americans have not yet been brought home.

“The United States owes a profound debt of gratitude to those American service members who gave their lives in service to their country and we are working diligently to bring them home,” Ms. Sanders said. “It is a solemn obligation of the United States government to ensure that the remains are handled with dignity and properly accounted for so their families receive them in an honorable manner.”

Earlier this week, Mr. Trump said the process of returning the remains was under way.

“As you may know, we’re also working to bring back the remains of your brothers-in-arms who gave their lives in Korea. And I hope that, very soon, these fallen warriors will begin coming home to lay at rest in American soil. That’s starting the process,” the president said during a speech to a VFW convention in Kansas City.

Following the repatriation ceremony in South Korea, the remains are expected to transported to Hawaii for forensic identification, the Yonhap News Agency reported.