On Sunday, North Korean state media showed leader Kim Jong Un observing live-fire drills of long-range multiple rocket launchers (Picture: PA).
North Korea has fired at least one unidentified projectile from its western area, according to the South Korean military.
It is thought to be the second such launch in the past five days.
The South's Joint Chiefs of Staff had no other immediate details of the Thursday afternoon launch.
It came hours after the North through its state media described its earlier firing of rocket artillery and an apparent short-range ballistic missile as a regular and defensive military exercise.
On Saturday, South Korea's Joint Chiefs of Staff said it detected North Korea firing multiple projectiles toward the sea from near the eastern town of Wonsan.
The following day, North Korean state media showed leader Kim Jong Un observing live-fire drills of long-range multiple rocket launchers and what appeared to be a new short-range ballistic missile fired from a launch vehicle.
Pyongyang said criticism from South Korea about the launch was a "cock-and-bull story".
Defence representatives from South Korea, the United States and Japan are expected to meet in Seoul to discuss the North Korean launches and other security issues.
South Korea's presidential Blue House and defence ministry have raised concern that Saturday's launches went against the spirit of an inter-Korean military agreement reached last year to cease all hostile activities.
The agreement urged North Korea to refrain from acts that could escalate tensions.
The launches likely represented North Korea's first ballistic missile launch in more than 500 days and were clearly a sign of Pyongyang's frustration at stalled diplomatic talks with Washington meant to provide coveted sanctions relief in return for nuclear disarmament.