North Korean leader Kim Jong Un has agreed to "cooperate closely" with the U.S. — even possibly accepting "stronger" denuclearization measures — a top South Korean official said Thursday, amid an apparent rift over progress in U.S.-North Korean nuclear talks.

South Korean President Moon Jae-in's national security chief, Chung Eui-yong, who led a special delegation to the North Korean capital a day earlier to meet Kim, told a televised news conference that the two sides had also set the date for Moon's visit to Pyongyang for Sept. 18 to 20. Ahead of that trip, Chung said, the North and South would open a joint liaison office at the Kaeseong industrial complex north of the border.

At the summit — their third — the two leaders are scheduled to discuss measures on denuclearization, establishing peace on the peninsula, and detailed steps to help ease military tensions, Chung added. Such a summit would make Moon the first South Korean leader to visit Pyongyang in 11 years.