North Korea launched three short-range guided missiles into the Sea of Japan on Saturday, Yonhap news agency reported, quoting South Korea's Defense Ministry.
The ministry said it detected two launches Saturday morning, followed by another in the afternoon.
In Tokyo, a Japanese government official confirmed the launches and said the missiles did not fall in Japanese waters.
Yonhap quoted a South Korean government official as saying, "A more detailed analysis will be needed but the missiles launched may be a modified anti-ship missile or the KN-02 surface-to-surface missile derived from the Soviet era SS-21 that has a range of about 120 kilometers."
The official said judging by the trajectory and distance traveled, the missiles fired were not medium- or long-range ballistic missiles.
Following the missile firings, South Korea stepped up monitoring of North Korea and is on high-level alert to deal with any developments, Yonhap said.
In April, North Korea deployed two Musudan intermediate-range missiles along with medium-range Rodong missiles on its east coast in an apparent countermeasure against joint South Korea-U.S. military exercises being held at the time.
However in early May, Pyongyang took the Musudan missiles off launch-ready status and moved them away from the east coast, according to diplomatic sources.
The Musudan has an estimated range of up to 4,000 km, giving it the ability to reach the U.S. territory of Guam, while the Rodong with a reach of 1,500 km could strike anywhere in South Korea and parts of Japan.
The Musudan missile was first displayed in public during a military parade in Pyongyang in October 2010.