A file image of a North Korean missile launch is shown during a news program being watched on a TV screen at Seoul Station, July 19. AP-Yonhap

North Korea fired several cruise missiles into the Yellow Sea on Saturday, according to the Joint Chiefs of Staff (JCS).

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.

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It also came amid heightened tensions on the Korean Peninsula following the first 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.

The USS Kentucky (SSBN 737), an 18,750-ton Ohio-class nuclear ballistic missile submarine (SSBN), departed Busan Naval Base, Friday, three days after a rare port visit in a major show of strength against evolving North Korean military threats.

Pyongyang 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 and two "Hwasal-2"-type strategic cruise missiles, launched from South Hamgyong Province, accurately hit targets set in the East Sea. (Yonhap)