The South Korean military says North Korea is believed to have fired artillery rounds.

The Joint Chiefs of Staff said they had detected "trajectories" believed to be artillery shots from around 6:21 p.m. to 6:37 p.m. on Sunday. They disclosed no further details, including how long they flew.

South Korea's Yonhap News Agency reports that about two shots are believed to have been fired from the western side of North Korea toward the Yellow Sea located to the west of the Korean Peninsula.

The South Korean presidential office says National Security Adviser Kim Sung-han received a briefing from the Joint Chiefs of Staff and confirmed the military's posture. The military is boosting its surveillance and vigilance.

Earlier, the South Korean defense ministry announced that six US F-35A stealth fighter jets arrived in South Korea last Tuesday to conduct joint drills through Thursday of this week.

South Korean media report that North Korea began its military exercises this month. They also report that Pyongyang may have fired the shots in protest at the US-South Korea joint drills.