A North Korean missile production facility in the city of Hamhung is seen from a satellite image taken on April 1, 2018. Yonhap

North Korea has "significantly" expanded a key long-range missile base, while denuclearization negotiations with the United States have stalled, CNN reported Wednesday, citing new satellite imagery.

It's the latest in a series of reports suggesting that North Korea continues to develop its missile programs short of an agreed plan with the U.S. to dismantle them.

The base is identified in a report by the Middlebury Institute of International Studies at Monterey as being located in the mountainous interior of the country, CNN said. In addition to the known base in Yeongjeo-dong, there is also construction on a previously unreported site 7 miles away, which could be separate or subordinate to the old site, it said.

"Satellite images show that the base remains active," the report says, according to CNN. "Moreover, in the past year North Korea has significantly expanded a nearby facility that appears to be another missile base."

A visitor looks around models of mock North Korea's Scud-B missile, right, and other South Korean missiles at Korea War Memorial Museum in Seoul, South Korea, Tuesday, Nov. 4, 2008. Yonhap

The photos were taken in October and November, after U.S. President Donald Trump and North Korean leader Kim Jong-un agreed at their historic June summit to work toward the "complete" denuclearization of the Korean Peninsula.

The newly identified site's "unique location makes it a strong candidate to receive North Korea's newest long-range missiles, including those that can carry nuclear weapons and can strike the United States," CNN quoted the report's authors as saying.

The base was one of several the U.S. sought access to as part of an agreement with North Korea in 2000, CNN said, adding that Kim's father and then leader Kim Jong-il refused to comply.

The report also describes a pair of hardened "drive-through" shelters covered with soil and trees as a disguise, and five entrances to underground tunnels that could be used to store missiles. (Yonhap)