Authorities said North Korea has fired two short-range missiles into the sea, in an apparent protest against joint U.S. and South Korean military exercises launched Monday.
South Korea's Yonhap news agency quoted officials as saying the missiles fell into the East Sea 490 kilometers from the North Korean port city of Nampo.
South Korean Defense Ministry spokesman Kim Min-seok said the incident was considered "a deliberate provocation seen as an armed protest" as the North fired the missiles without designating a no-sail zone.
Kim told reporters: "Our military and U.S. forces have enhanced surveillance posture and we are maintaining a posture to immediately respond. If North Korea takes provocative actions, our military will react firmly and strongly so North Korea will regret bitterly.''
North Korea has long demanded that Washington and Seoul stop the annual military drills, which Pyongyang has called "a dress rehearsal for a northward invasion."
Pyongyang routinely uses missile tests and artillery fire to protest the joint military maneuvers. It also has used such launches, as well as three nuclear tests in recent years, to defy international critics and push back against U.N. sanctions levied against the North for its ballistic missile launches.
Japanese officials also registered their concern.
Chief Cabinet Secretary Yoshihide Suga said, "North Korea's missile launches are a major problem. From the point of view of safety issues for flights and shipping vessels but also a transgression of the North Korea-Japan Pyongyang Statement, the joint statement of the six-way talks and the related (U.N.) Security Council resolutions. We have lodged a severe condemnation with North Korea."