WASHINGTON (AP) — U.S. analysts say they have located more than half of an estimated 20 secret North Korean missile development sites.

The findings come as the Trump administration’s denuclearization talks with the North appear to have stalled. And they highlight the challenge the U.S. faces in ensuring that North Korea complies with any eventual agreement that covers its nuclear and missile programs.

The report released Monday by the Center for Strategic and International Studies draws on commercial satellite imagery. The authors say their analysis identifies 13 of the secret facilities used to produce missiles and related technology.

Although the sites are not launch facilities, the authors say they must be declared by North Korea and inspected in any credible, verifiable deal.