SEOUL - North Korean troops pledged Monday to defend leader Kim Jong-Un with their lives after his uncle was executed for an alleged plot, as South Korea put its forces on alert for any “reckless provocations”.

The mass rally in Pyongyang came ahead of Tuesday’s second anniversary of the death of longtime leader Kim Jong-Il, whose sudden demise thrust his young son Jong-Un to the helm of the secretive state.

Kim has apparently been trying to demonstrate his firm grip on power following the shock execution Thursday of his uncle Jang Song-Thaek.

The deadly purge prompted both Seoul and Washington to warn that vigilance is needed against any surprises by the nuclear-armed regime.

State TV showed thousands of troops marching in a large square outside the Kumsusan Palace of the Sun, which houses the embalmed bodies of the country’s founder Kim Il-Sung and his son Jong-Il.

“Let’s become rifles and shields to safeguard the supreme commander!” they chanted during a memorial rally for Jong-Il. Choe Ryong-Hae, head of the military’s political bureau, urged the soldiers to protect Kim Jong-Un “at the cost of their lives” and become “human bullets and bombs” for him. Choe, seen as one of the most powerful military figures after Jang’s execution, also said North Korean troops, undeterred by “strong winds”, would defend the young leader to death despite the “manoeuvring of betrayers”.

A show of public support from the military is likely even more important following the execution of Jang, a veteran fixer and political eminence grise who was seen as playing a key role in bolstering the leadership of the young and inexperienced Kim.

The South’s President Park Geun-Hye called a meeting of top defence and national security officials. “Given the latest development in the North, it is uncertain in what direction its political situation would evolve,” she told the meeting.

“We also can’t rule out the possibility of contingencies such as reckless provocations,” she said, urging the military to step up vigilance along the heavily fortified border.

On Monday thousands of North Korean propaganda leaflets fell on the frontline island of Baengnyeong warning of an attack on South Korean soldiers stationed there, according to the South’s Yonhap news agency.

The South’s military declined to confirm the report. The North shelled another frontline island in November 2010 and killed four people including civilians.

Park discussed what she called the “grave and unpredictable” situation at the meeting.

“She ordered (officials) to strengthen the joint defence posture with the US... and continue to closely coordinate and share intelligence with related countries and the international community,” her spokesman said.

Washington called for a united international front against North Korea by countries including China, North Korea’s main patron. Secretary of State John Kerry said Jang’s execution showed the world “how ruthless and reckless” Kim is, and voiced 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 aired Sunday.

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 denuclearisation as possible”, he added.

Jang’s execution - just days after he was ousted from all his party and military positions - marks the biggest political upheaval since the young Kim inherited power.

The purge was staged in an extraordinarily public and dramatic manner, with Pyongyang releasing images of Jang being dragged out of a party meeting. Another image showed a handcuffed Jang being held by uniformed guards at the military tribunal that sentenced him to death.

Jang had been seen as Kim’s political mentor but the 67-year-old’s growing political influence and power was increasingly resented by the young leader, analysts said.

Kim spent a whirlwind weekend making public appearances around the country in a move apparently intended to show he is firmly in charge.

State media have reported since Saturday his “field guidance” trips to venues including a luxury ski resort and a military fish warehouse, as well as a wake for a late senior party official.

He was photographed smiling and laughing with top military cadres who accompanied him.

“I think Kim is trying to boast that he’s fully in control of the regime and reassure the North’s public that the regime is stable even after Jang’s death,” said Shin In-Kyun, head of the Korea Defence Network think tank.