South Korea's Joint Chiefs of Staff and the Defense Ministry said they were checking on the report.
Yonhap said the North had showed signs of preparations earlier this week for testing short-range missiles as part of routine training, declaring a no-sailing zone off the coastal city of Nampo and placing a military boat equipped with anti-ship missiles on standby there.
But the North appeared to have dropped the plan, after it did not fire a missile at that time, Yonhap said.
The reported launches come a day after South Korea withdrew officials from a joint industrial zone with North Korea on Pyongyang's request.
The move was sparked by the North's anger over the tougher policy stance taken by new South Korean President Lee Myung-bak on the communist nation, a contrast from a decade of liberal Seoul governments.
The North regularly test fires missiles, and its long-range models are believed able to possibly reach the western coast of the United States. The country also conducted its first-and-only nuclear bomb test in October 2006, but it is not known to have a weapon design able to fit inside a missile warhead.
