Fifty-five cases containing what were believed to be the remains of U.S. servicemen killed during the Korean War were returned Friday, the White House said.

The cases, each draped with the flag of the United Nations, arrived aboard a U.S. military plane at Osan Air Base outside Seoul, South Korea. Earlier, the plane and its crew had traveled to Wonsan, North Korea, to collect the remains, the White House said.

The transfer of the remains from the 1950-53 Korean War had been negotiated last month during a summit meeting in Singapore between President Trump and North Korean leader Kim Jong Un.

President Donald Trump tweeted about the return of the remains late Thursday, as the transfer mission was getting underway.