Tensions between Seoul and Pyongyang have soared since the South last month blamed an explosion inside the heavily fortified Demilitarized Zone (DMZ) separating the two countries on landmines laid by North Korea, an accusation it has rejected, AFP reported.
"Our military leadership underlined the need to make those rogues aware that our new firepower units have taken their firing positions," Kim Yo Jong said, according to state media.
Her statement came a day after Seoul warned of a "firm" response to further North Korean provocations.
She added that Kim Jong-un had "issued an order to fortify the border with an enemy state to be eternally impregnable".
The statement came as the Korea Central News Agency said the armed forces' Marshal Pak Jong Chon had inspected "pivotal positions" on the border, stressing to troops it was "time to organize the firepower strong enough to strike terror into the enemy's heart".
"If the enemy troops make a provocation actually threatening the safety of our service personnel... The border forces of the DPRK must take very proficient, specialized and overwhelming military action," he said, using the acronym for the North's official name.
Hong Sung-pyo, senior researcher at the Korea Institute for Military Affairs, told AFP on Wednesday that the claimed troop movements should be taken seriously.
"These are small military positions -- essentially guard posts or fighting positions for units armed with individual firearms -- and it looks like North Korea has been moving them forward," he said.
"Of the developments we have seen so far, this appears to be among the most serious. North Korea has conducted various demonstrations of its firepower, but this now appears to be about operational deployment," he added.
North Korea has separately tested a slew of weapons recently, including a "new-type" precision-guided torpedo system and an intermediate-range hypersonic missile, both of which were said to employ AI technology.
Seoul's military on Tuesday accused Pyongyang of attempting to shift the blame for escalating military tensions, while confirming the use of loudspeakers and warning shots as "legitimate defensive measures to prevent North Korean troops from crossing the MDL (Military Demarcation Line) and to drive back troops that have crossed it".
Jin Yong-sung, chairman of South Korea's Joint Chiefs of Staff, also ordered the military to "respond with firm resolve" should North Korea use Seoul's actions as a pretext for aggressive action.
Kim Yo Jong said a day earlier that the South's use of loudspeaker broadcasts and warning shots had "gone beyond the red line", while threatening the deployment of "more tactical strike means along the Southern border".