The launches coincide with joint exercises between the U.S. and South Korean militaries, exercises that North Korea always strongly protests because it considers them preparation for an invasion.
The first and third missiles failed in flight, while the second appears to have blown up almost immediately, said U.S. Pacific Command in Hawaii, which monitors North Korean missiles.
All three appeared to be short-range missiles, rather than the long-range types designed to be able to strike the United States, and were launched from Kittaeryong on North Korea's east coast.
"We are working with our interagency partners on a more detailed assessment and we will provide a public update if warranted," Commander Dave Benham, a spokesman for Pacific Command, said in a statement.
South Korea's Joint Chiefs of Staff said that a "projectile" had been fired on Saturday morning and that it was working to determine what kind of missile it was, according to the South's Yonhap News Agency. Early reports suggested that the missiles traveled only 150 miles before falling in the sea.
South Korea's president, Moon Jae-in, had been informed, Yonhap said.
Japanese authorities had determined they were ballistic missiles, Japan's NHK broadcaster and Kyodo News reported.
The Ulchi Freedom Guardian exercises, which mainly involve computer simulations rather than battlefield maneuvers or fly-overs with bombers, are currently taking place in South Korea.
North Korea this week said the exercises were evidence that the United States planned to invade North Korea.
"The reality vividly shows that the U.S. ambition for stifling [North Korea] remains unchanged no matter how much water may flow under the bridge and the puppet group's ambition for invading the north remains unchanged," the state-run Korean Central News Agency reported.
The United States and its "puppets" in South Korea should "act with discretion if they want to evade the historic moment of death," the statement said.
Heightening concerns about North Korea's aims, state media last week reported that leader Kim Jong Un visited the Chemical Material Institute of the Academy of Defence Sciences and inspected designs for two new long-range missiles.
Last month North Korea fired its first intercontinental ballistic missiles, technically capable of reaching the mainland United States, and this month warned that it was considering launching missiles into the Pacific Ocean near the American territory of Guam.
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