South Korea's president Tuesday warned Seoul would react "mercilessly to any provocations" from the North, describing the situation over the border as "ominous" as she visited a frontline guard post.
Seoul and Washington have been increasingly concerned over the stability of the North Korean regime following the execution two weeks ago of Jang Song-Thaek, a high-level official and uncle of young leader Kim Jong-Un.
"The security situation on the Korean peninsula is very grave. North Korea's internal situation is ominous, raising concerns about provocations," said South Korean President Park Geun-Hye, wearing a military uniform on her first visit to the tense border since taking office in February.
"We should react sternly and mercilessly to any provocations by North Korea," she said, calling for "watertight security readiness".
Her warning came as Kim, flanked by senior military officials, visited the mausoleum of his late father in Pyongyang to pay his respects on an important anniversary.
Kim visited the Kumsusan Palace of the Sun, which houses the embalmed bodies of his late father and grandfather, the Korean Central News Agency (KCNA) said.
Kim's father, Kim Jong-Il, was formally made the country's top military commander on December 24 in 1991 -- three years before the death of his own father and the country's founding president, Kim Il-Sung.
TV news footage showed Kim, clad in a dark Mao suit, standing before the white statues of his two predecessors, accompanied by dozens of uniformed military officials who bowed deeply towards the statues.
Among the top military cadres accompanying the young ruler were Choe Ryong-Hae, the director of the military's political department, and defence chief Jang Jong-Nam, KCNA said.
The visit came 12 days after Kim executed his once-powerful uncle in the biggest political upheaval since he took power after the death of his father two years ago.
Jang Song-Thaek, once the country's unofficial number two, was executed on December 12 after being accused of corruption and plotting a coup.
The shock purge -- staged in an unusually public and dramatic fashion -- has raised concerns about potential political instability in the isolated communist state.
Jang Song-Thaek, 67, played a key role in cementing the power of the inexperienced Kim, but his increasing political influence and power was resented by his nephew barely half his age, analysts said.
His growing control over the country's lucrative mineral trades drew ire of other top officials and played a role in his downfall, Seoul's intelligence chief said Monday.
The reclusive state's propaganda mill has since gone into overdrive describing Jang as a traitor while extolling Kim's leadership.
Tens of thousands of troops pledged loyalty to him in a mass rally on the death anniversary of his father last Tuesday.
The Kim dynasty has ruled the impoverished but nuclear-armed state since 1948 with an iron fist and pervasive personality cult.