North Korea fired 18 short-range rockets into the sea off its east coast Sunday, South Korean officials said, in an apparent continuation of protests against ongoing U.S.-South Korean military drills.

Such short-range rocket tests are usually considered routine, as opposed to North Korean long-range rocket or nuclear tests, which are internationally condemned as provocations. North Korea has conducted a string of similar short-range launches in recent weeks that have coincided with the annual military drills by allies Washington and Seoul.

North Korea says the drills are preparation for an invasion. The allies say the exercises, which last year prompted North Korean threats of nuclear war against the South and the United States, are routine and defensive in nature.