President Park Geun-hye on Friday expressed "very strong regret" over North Korea's artillery shelling near a South Korean patrol ship, calling for an airtight defence posture and further efforts to ensure the safety of residents in the region.
The incident took place late Thursday around 150 meters from the vessel sailing near the Northern Limit Line, a de facto inter-Korean border, prompting the South to shoot back five shells immediately. The Joint Chiefs of Staff reported no damage.
Park convened a meeting with top foreign and defence aides, instructing them to maintain a thorough defence posture and close cooperation with the US while intensifying coordination with China and other partners.
"The president displayed very strong regret that North Korea staged such a provocation at a time when our citizens are in grief in the wake of the sinking of the Sewol ferry," Cheong Wa Dae said in a statement.
"She also directed them to make utmost efforts to protect the people including those living in the northwestern island areas so that they do not feel uneasy."
Pyongyang, for its part, accused the South's navy of breaching the frontier and initiating a provocation.
The Rodong Sinmun, a mouthpiece of the ruling Workers' Party, lambasted the South for a "deliberate, premeditated provocation," warning that the North's army and people were "fully ready and only awaiting an order of an attack."
"(Seoul's claim was) a deceitful play to justify a preposterous lie and decry our army as a warmonger," the Southwestern Front of the Korean People's Army said in a statement carried by the official Korean Central News Agency.
The South's Defence Ministry dismissed the assertion as a "blatant lie" intended to evade responsibility for the provocation.
Earlier Thursday, the South Korean Navy issued a strong warning to the communist neighbour over a recent series of violations of the NLL, including by three military boats on Tuesday that pulled back upon warning shots.
Pyongyang has persistently tried to invalidate the NLL, arguing that it was unilaterally drawn by the US-led UN Command after the end of the 1950-53 Korean War and should be moved further south.
"We've already warned sternly against the North's provocations through a telephone message and made clear that we will respond adamantly if provocations continue," ministry spokesman Kim Min-seok told a news briefing.
"Our military is strengthening our surveillance and operation readiness and maintaining a firm readiness posture."
US State Department spokesperson Jen Psaki also urged Pyongyang to "refrain from provocative actions that aggravate tensions" and take steps to improve its relations with its neighbours.