SEOUL, South Korea (AP) — U.S. and North Korean officials held “productive” talks Sunday to discuss the return of U.S. service members’ remains missing since the Korean War, U.S. Secretary of State Mike Pompeo said.

Pompeo, who was not part of the talks, said in a statement that working level meetings between U.S. and North Korean officials would begin on Monday “to coordinate the next steps, including the transfer of remains already collected” in North Korea.

It was not immediately clear who took part in Sunday’s talks, held at the tense inter-Korean border. Pompeo said they were “the first General Officer-level talks” with North Korea since 2009.

He said the meeting “was aimed at fulfilling one of the commitments” made by North Korean leader Kim Jong Un at last month’s summit with President Donald Trump in Singapore.

“Today’s talks were productive and cooperative and resulted in firm commitments,” Pompeo said.

“Additionally, both sides agreed to re-commence field operations in the DPRK to search for the estimated 5,300 Americans who never returned home,” he said. North Korea is officially known as the Democratic People’s Republic of Korea.

North Korean officials skipped a planned meeting with U.S. officials over the war remains last week, citing lack of preparations. South Korea’s Foreign Ministry said the North then requested higher level talks with the U.S.-led United Nations Command.

There’s speculation that Pyongyang is trying to fast-track discussions on more critical issues, such as reaching a declaration to formally end the war, which stopped on an armistice and not a peace treaty.

Sunday’s meeting came a week after Pompeo traveled to Pyongyang to begin follow-up talks to last month’s Singapore summit in which Trump and Kim issued vague aspirations for a nuclear-free Korean Peninsula without describing when and how it would occur.

Last month, the U.S. military moved 100 wooden coffins to the inter-Korean border to prepare for the return of U.S. war remains, which was a rare tangible commitment the North made during the Trump-Kim summit.

About 7,700 U.S. soldiers are listed as missing from the Korean War, and 5,300 of the missing are believed to be in North Korea. The war killed millions, including 36,000 American soldiers. Richard Downes, executive director of the Coalition of Families of Korean & Cold War POW/MIAs, said recently that he had been told the North may have the remains of more than 200 American service members that were likely recovered from land during farming or construction.

Efforts to recover U.S. war remains have been stalled for more than a decade due to tensions over North Korea’s nuclear program and a previous U.S. claim that security arrangement for its personnel working in the North was insufficient.

From 1996 to 2005, joint U.S.-North Korea military search teams conducted 33 recovery operations that collected 229 sets of American remains. The last time North Korea turned over remains was in 2007, when Bill Richardson, a former U.N. ambassador and New Mexico governor, secured the return of six sets.