North Korea launched 30 short-range rockets into the sea off its east coast Saturday, South Korea said, in the latest in a series of apparent protests against ongoing U.S.-South Korean military drills.

Such short-range launches are normally seen as routine, but there have been an unusual number this year coinciding with the annual, routine war drills by Washington and Seoul that North Korea claims are invasion preparation. Analysts say the impoverished North chafes against the annual drills because it has to spend precious resources responding with its own exercises. As part of those drills, about 13,000 U.S. and South Korean forces on Thursday were to begin a series of amphibious landings.

Maj. Kim Nam-wook of the South Korean Defense Ministry said the North Korean rockets flew about 37 miles early Saturday morning. It wasn’t immediately clear what kind of rockets were fired.