The U.S. called for a united front against North Korea after the "ominous" execution of leader Kim Jong-Un's uncle as South Korea put its forces on alert for any "provocations" from its nuclear-armed neighbor, the Agence France-Presse reported.
The warning came as thousands of North Korean troops marched in Pyongyang to pledge their loyalty to the young leader and mark the second anniversary of the death of Kim's father and longtime leader Kim Jong-II.
According to AFP, the rally was held days after North Korea executed Jang Song-Thaek on Thursday. Song-Thaek has long been seen as the country's unofficial number two and Kim Jong-Un's political regent. He was executed for reportedly plotting to overthrow his nephew and corruption.
Since Kim took power after the death of his father two years ago, the execution has been the biggest political upheaval, AFP reported.
U.S. Secretary of State John Kerry said the purge showed the world "how ruthless and reckless" Kim Jong-Un is, voicing concerns over the nuclear weapons under the control of the "spontaneous, erratic" leader.
"It's an ominous sign of the instability and of the danger that does exist," Kerry said in an interview with ABC television during a trip to Vietnam. "To have a nuclear weapon, potentially, in the hands of somebody like Kim Jong (Un) -- just becomes even more unacceptable."
Last week's events showed the urgent need to get "China, Russia, Japan, South Korea, all of us to stay on the same page and to put as much effort into the denuclearization as possible," Kerry said.
South Korea was on alert for potential fallout from the purge north of the border as President Park Geun-Hye warned of possible "reckless provocations" by Pyongyang.
"Given the latest development in the North, it is uncertain to what directions its political situations would evolve," Park said in a meeting with advisers Monday. "We also can't rule out the possibility of contingencies such as reckless provocations."
Park discussed what she called the "grave and unpredictable" situation in a high-level meeting with top defense and national security officials including ministers handling intelligence and North Korean affairs, AFP reported.
"She ordered (officials) to strengthen the joint defense posture with the US and continue to closely coordinate and share intelligence with related countries and the international community," Park's spokesman said.
