A U.S. think tank says North Korea is secretly operating a missile base in Chagang Province, some 240 kilometers north of the Demilitarized Zone.

The assessment was shared Thursday by Beyond Parallel, a website on the North run by the Center for Strategic and International Studies(CSIS).

The website said the Yongnim Missile Operating Base is believed to be one of up to 20 operational or abandoned ballistic missile bases in North Korea.

It said the base likely houses a regiment or brigade-sized unit equipped with intermediate-range ballistic missiles or intercontinental ballistic missiles.

Construction of the base reportedly began around 1994 by the Military Construction Bureau, suggesting a considerable level of advance planning, according to CSIS.

CSIS also suggests the base was an early component of the North’s strategic missile belt, part of the regime’s “extensive and growing nationwide dispersed ballistic missile network.”