INA- sources
So far this year, Kim has declared South Korea his country's "principal enemy", jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.
Pyongyang has also ramped up weapons tests, including cruise missiles, an "underwater nuclear weapon system" and a solid-fuelled hypersonic ballistic missile.
"Our military detected several unknown cruise missiles over the waters northeast of Wonsan around 09:00 today (1200 GMT) and South Korea-US intelligence authorities are conducting a detailed analysis," the Joint Chiefs of Staff said in a statement.
It added that the military was "strengthening surveillance and vigilance, and closely watching for any additional signs and activities from North Korea".
Earlier this week, North Korea announced it had tested a new control system for a multiple rocket launcher it said would have an "increased" battlefield role.
The cruise missile launches have prompted speculation from experts that it is testing the weapons before shipping them to Moscow for use in Ukraine.
Pyongyang and Moscow have bolstered ties in recent months, with leader Kim Jong Un making a rare trip to Russia to meet President Vladimir Putin in September.
Seoul and Washington have accused the North of supplying Russia with weapons in exchange for technical support for Kim's nascent satellite programme, which would violate a raft of UN sanctions on both regimes.
Unlike their ballistic counterparts, the testing of cruise missiles is not banned under current UN sanctions on Pyongyang.
Cruise missiles tend to be jet-propelled and fly at a lower altitude than more sophisticated ballistic missiles, making them harder to detect and intercept.
"It is believed that North Korea exported large quantities of multiple rocket launchers to Russia last year," Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.
He added that there may have been quality issues with some of the exported weapons and that the recent spate of testing could be leader Kim "taking action to address the issue".
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