North Korean leader Kim Jong-Un has warned war could break out "without any prior notice" and urged his military to bolster its combat readiness, state media reported.
The call on Wednesday comes one day after a US-Korea Institute at Johns Hopkins University report said that satellite imagery suggested that the North might have begun producing fuel rods for its recently restarted nuclear reactor.
There has been heightened tension on the Korean Peninsula following the execution of Kim's uncle and former mentor in an unusually public purge.
Seoul and Washington have warned of possible provocative acts by the nuclear-armed North following the execution of Jang Song-Thaek, a senior leader who was also the uncle and former political mentor of the younger leader.
Kim visited the Command of Large Combined Unit 526 on Christmas Eve, the North's official Korean Central News Agency said.
"He instructed the unit to put utmost spurs on rounding off its combat readiness... always bearing in mind that a war breaks out without any prior notice," it said.
The unit is based in the North's western port city of Nampo, according to the South's Yonhap news agency.
The Johns Hopkins University report said that satellite imagery had identified facilities at the Yongbyon Nuclear Scientific Research Centre that might produce fuel for North Korea’s recently restarted plutonium production reactor and the Experimental Light Water Reactor still under construction.
"The identification of these facilities indicates a more wide-ranging, extensive effort by North Korea to modernise and restart the Yongbyon complex dating back to 2009 than previously understood," the report said.
South Korean President Park Geun-Hye called for "watertight security readiness" during her trip on Tuesday to a frontline guard post, as she described the situation over the border as "ominous".
"We should react sternly and mercilessly to any provocations by North Korea," she said.
The reclusive state's propaganda mill has gone into overdrive in recent days, 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.