Seoul, Jul 22 (IANS): North Korea fired several cruise missiles into the Yellow Sea on Saturday,  amid heightened tensions on the Korean Peninsula following the port visit of a nuclear-capable US submarine in more than 40 years and the inaugural session of the South Korea-US Nuclear Consultative Group earlier this week.

South Korean and US intelligence authorities were analysing 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, Yonhap news agency reported.

"Our military has bolstered surveillance and vigilance while closely cooperating with the US and maintaining a firm readiness posture," the Joint Chiefs of Staff (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.

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.