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.

After international condemnation of its third nuclear test last year, North Korea responded to the drills with threats of nuclear war against the South and the United States. It has toned down its rhetoric this year. Analysts say that's because it believes better ties with Seoul might win it much-needed aid and outside investment to help its crumbling economy.

A week ago, North Korea launched 25 rockets with a range of about 44 miles, South Korea said, and earlier this month a North Korean artillery launch happened minutes before a Chinese commercial plane reportedly carrying 202 people flew in the same area.

Pyongyang has said that its recent rocket drills are part of regular training and that officials are mindful of international navigation.

The Korean Peninsula remains officially at war because the Korean War ended with an armistice, not a peace treaty.