The South Korean military says a North Korean long-range missile base which CNN reported has continuously expanded amid diplomatic efforts to denuclearize the North, is a key site that it is monitoring in cooperation with the U.S.

At Thursday's briefing, the Joint Chiefs of Staff spokesperson Army Colonel Roh Jae-cheon declined to comment further, saying it would not be appropriate for the military to officially acknowledge the content of a foreign media report.

However, an official at the military, said the facility in the North's Yeongjeo-ri is known to be a base for medium-range Nodong missiles and has long been subject to Seoul and Washington's tracking and surveillance.

Citing new satellite images, CNN reported the Yeongjeo-ri missile base and a nearby, previously unreported site in the North's mountainous interior remain active and have been continuously upgraded.