Secretive North Korea raised its military alert ahead of a live-fire artillery drill by rival South Korea on Monday amid heightened tension on the peninsula during a delicate transition of power in the impoverished North.

The exercise, involving the use of mortars, some 5,000 rounds of ammunition and attack helicopters, took place near a disputed sea border off the west coast and near a South Korean island bombarded by the North after a similar drill in 2010. South Korea said the drill, the second of its kind in the area this year, was routine and had passed without incident.

“The exercise took place normally according to plan. North Korea maintained a higher level of response posture than normal,” the South’s Yonhap news agency quoted a military official as saying.

The North’s new young leader, Kim Jong-un, has taken a militaristic line in what analysts say is clearly an attempt to woo the backing of the powerful army as he tries to cement his grip on power as the third generation of the Kim dynasty.

Pyongyang had threatened “merciless retaliatory strikes” if the South violated its territorial waters during the exercise.