North Korea launched three short-range missiles into the Sea of Japan on Saturday, the South Korean Defense Ministry said.
The ministry detected two launches Saturday morning, followed by another in the afternoon, a ministry spokesman said.
In Tokyo, a Japanese government official confirmed the launches and said the missiles did not fall in Japanese waters.
Yonhap news agency 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.
A South Korean Defense Ministry official said the launches could be part of a North Korean military exercise or tests of a newly developed land-to-ship missile.
Following the missile firings, South Korea stepped up monitoring of North Korea and is on high-level alert to deal with any developments, the South Korean Defense Ministry said. North Korea last launched short-range missiles on March 15.
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.