South Korea's presidential office has confirmed that top U.S. envoy on North Korea Stephen Biegun was still in Pyongyang as of Friday morning.

Presidential spokesperson Kim Eui-kyeom denied local media reports that Biegun returned to Seoul Thursday night after working-level discussions with North Korean officials. He was referring to reports that a plane, likely carrying some 20 people including Biegun, landed at the U.S.' Osan Air Base from Pyongyang late Thursday.

Kim said the plane that arrived late Thursday likely transported people and supplies, but that he was certain Biegun was still in Pyongyang.

The spokesperson also said he was unable to specify when the U.S. envoy would return to Seoul and whether he had met with North Korean leader Kim Jong-un.

Biegun, who left Seoul for Pyongyang on Wednesday for talks with his North Korean counterpart Kim Hyok-chol to prepare for the second U.S.-North Korea summit later this month, is expected to return to Seoul as early as Friday.

The U.S. envoy plans to brief South Korean officials on the outcome of his discussions in Pyongyang and exchange views on future negotiations.