North Korea is yet to confirm the suspected test-launch of two missiles on Wednesday morning.
State-run KCNA news agency told Xinhua that it was not aware of the missile launches.
South Korea's Yonhap news agency quoted the Joint Chiefs of Staff as saying that North Korea launched a second missile, which was believed to be a Musudan, at about 8.05 a.m. from Wonsan on the east coast, two hours after Pyongyang test-fired a ballistic missile, which Seoul said was presumably a failure.
Latest launch was the sixth test-firing of its kind by Pyongyang after the five earlier launches were all believed to have failed.
South Korean army said it is not known if Pyongyang's second launch on Wednesday was a success or not.
It is believed that a ballistic missile is required to fly at least 300 km to be considered successful in test-firing. The second missile launched on Wednesday flew about 400 km.
First test-launch on April 15 failed as the missile blasted in mid-air several seconds after take-off.
April 28 launch also failed as those blasted in mid-air or crashed in waters. The fourth test was expected to have exploded on its mobile launcher even before take-off, as per the South Korean military.