South Korean and U.S intelligence authorities were analyzing the launches, which took place at about 4:00 a.m., to learn more about the type of missiles fired and other details, according to the JCS.
"Our military has bolstered surveillance and vigilance while closely cooperating with the United States and maintaining a firm readiness posture," the JCS said, adding it is closely monitoring additional activities by the North.
The launches came three days after North Korea fired two short-range ballistic missiles into the East Sea.
It also came amid heightened tensions on the Korean Peninsula following the port visit of a nuclear-capable U.S. submarine in more than 40 years and the inaugural session of the South Korea-U.S. Nuclear Consultative Group earlier this week.
USS Kentucky (SSBN 737), an 18,750-ton Ohio-class nuclear ballistic missile submarine (SSBN), departed Busan naval base on Friday, three days after a rare port visit in a major show of strength against evolving North Korean military threats.
North Korea denounced the visit, saying the SSBN's deployment may fall under the legal conditions for its use of nuclear weapons.
In March, North Korea said it launched strategic cruise missiles "tipped with a test warhead simulating a nuclear warhead."
At that time, the North's state media said two "Hwasal-1"-type strategic cruise missiles and two "Hwasal-2"-type strategic cruise missiles, launched in South Hamgyong Province, accurately hitting targets set in the East Sea.
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