"Our military is closely monitoring for provocative movements by North Korea and is maintaining all readiness postures," a statement from the military said.
The missile, launched near the city of Kusong, flew 700 kilometers (435 miles), the South Korean military said. A US defense official confirmed that it flew that far, but said the US is still assessing what type of missile it was.
A second defense official told CNN: "At this point, we see nothing consistent with an (intercontinental ballistic missile) launch."
