North Korea conducted a test of a large liquid-fueled rocket engine in late April, an analysis of imagery by a U.S. think tank has shown, as Pyongyang appears to be gearing up for another spy satellite launch.

The Center for Strategic and International Studies’ Beyond Parallel program said knowledgeable sources and an analysis of commercial satellite imagery taken on April 29 of the Yunsong engine test site at the North’s Sohae satellite launching station showed burned vegetation and scarring of the ground, suggesting an unannounced test had been conducted.

“This test is a clear indication that North Korea is continuing to pursue the development of liquid propellent rocket engine technology for both its ballistic missile and SLV programs,” the analysis said, referring to space launch vehicles.