North Korea tested what appeared to be multiple rocket launcher systems (MLRS) from near the country’s west coast on Sunday, according to South Korean authorities, in what one expert assessed could be a part of the DPRK’s summer military training.

The South Korean military detected “trails” from the artillery from 6:21 p.m. to 6:37 p.m. KST, Seoul’s Joint Chief of Staff (JCS) said, about a month after reporting a similar MLRS test on a Sunday.

“[The South Korean military] has strengthened surveillance and vigilance, while maintaining a thorough readiness posture based on close ROK-U.S. cooperation,” JCS said in a statement.