North Korea launched several ballistic missiles toward the Yellow Sea on Monday afternoon, Seoul's military said, on the first day of a large-scale joint U.S-South Korea military exercise on the Peninsula.

"South Korean military has increased surveillance and maintaining readiness posture in close cooperation with the U.S.," South Korea's Joint Chiefs said on Monday.

South Korea's military says North Korea has fired several ballistic missiles into the sea. The launch Monday happened hours after South Korean and U.S. troops kicked off their large annual combined drills,