The broadcasts will draw a furious response from North Korea, which considers them an act of psychological warfare, AP reported.
Pyongyang is extremely sensitive to any outside criticism of the authoritarian leadership of Kim, the third member of his family to rule. When South Korea briefly resumed propaganda broadcasts in August after an 11-year break, Seoul says the two Koreas exchanged artillery fire, followed by threats of war.
South Korea's Yonhap news agency reported that frontline troops, near 11 sites where propaganda loudspeakers started blaring messages at noon (0300 GMT), were on highest alert.
North Korea didn't immediately react, but its response could be especially harsh because of the high emotions surrounding the likely birthday of Kim, who is believed to be in his early 30s.
The broadcasts came as world powers sought to find other ways to punish the North for its widely disputed claim to have conducted its first hydrogen bomb test.
At a UN Security Council emergency session, diplomats pledged to swiftly pursue new sanctions against North Korea, saying its test was a 'clear violation' of previous UN resolutions. North Korea is already heavily sanctioned.