SEOUL, South Korea — North Korea has fired 18 short-range rockets into the sea in an apparent continuation of protests of ongoing U.S.-South Korean military drills.

Such short-range rocket tests are usually considered routine, as opposed to long-range rocket or nuclear tests, which are internationally condemned as provocations. Pyongyang has conducted a string of similar short-range launches recently coinciding with the annual military drills, which it claims are invasion preparation.

South Korean Defense Ministry spokesman Kim Min-seok says the type of rockets launched Sunday off the country’s east coast isn’t yet clear.

Earlier this month, Seoul said a North Korean artillery launch happened minutes before a Chinese commercial plane reportedly carrying 202 people flew in the same area.

Pyongyang says its rocket drills are part of regular training and are mindful of international navigation.