In a statement, Seoul's Defence Ministry expressed grave concern over Pyongyang's threats to deploy additional tactical strike weapons along the heavily armed border and resume loudspeaker broadcasts towards the south.
The escalation and exchange of threats has raised the possibility of clashes and undercuts diplomatic gains made by South Korean President Lee Jae Myung soon after he took office.
The ministry said broadcasts and warning shots by South Korea near the military demarcation line were a "legitimate defensive measure" aimed at preventing incursions and forcing North Korean troops who crossed the line to withdraw.
"If North Korea uses our legitimate measures as a pretext to carry out further provocations, we will punish it resolutely," the ministry said, urging Pyongyang to halt incursions and respond to efforts to resolve differences over the line.
North Korea has been installing mines and barriers along the border since April 2024.
Seoul's joint chiefs of staff chairman Jin Young-seung blamed North Korea for landmines south of the line that seriously wounded South Korean soldiers last month.
He called the incident "a clear provocation" and a violation of the armistice agreement inked in 1953, which ended three years of Korean War fighting and created a 4km demilitarised zone between the opposing forces, requiring both sides to retreat 2km from the line.
The warnings from Seoul came after North Korea's Kim Yo Jong, sister of leader Kim Jong Un, accused the South Korean military of endangering North Korean soldiers by firing warning shots during Pyongyang's work to fortify the border.
Her threat to revive North Korean loudspeaker broadcasts risks reversing a key de-escalation measure that President Myung has touted as an early achievement of his efforts to restart diplomacy with Pyongyang.
Analysts said the stand-off increased the risk of a military clash.
"If North Korea's threshold falls below South Korea's existing rules of engagement, the simple repetition of current procedures could be enough to create the conditions for a clash," Korea Institute for National Unification senior research fellow Hong Min said.
Michael Bosack, founding director of the Parley Policy Institute and a former UN Command official, said troops could mistake warning shots for missed attacks, risking "misinterpretation of intent and potential counterfires."
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