North Korean soldiers have crossed the inter-Korean armistice line into South Korea five times this year as part of their beefed-up ambush infiltration training, military sources here said Tuesday.

"In accordance with the order by its leader Kim Jong-un, North Korea has been proactively carrying out their drills for surprise attacks and border intrusion inside the Demilitarized Zone, or DMZ, areas, this year," a military source said. He declined to be identified.

"Over the course of the training, soldiers from the North crossed the military demarcation line (MDL) five times this year.

Spotting them, we fired warning shots and expelled them," the source added.

Hiding in an underground tunnel inside the DMZ during the day, the soldiers from the communist country have in general launched their operations at night, he noted.

The DMZ is a 259-kilometer strip of rugged no-man's land stretching from coast to coast, strewn with land mines and barbed wire. The MDL runs along the middle of DMZ and was established after the agreement that ended the 1950-53 Korean War.

In the latest case, three North Korean soldiers infiltrated into the DMZ area under the South's jurisdiction on June 19 and came some 600 meters away from a general post in the city of Paju, just north of Seoul, to break a bell that South Korea set up for North Koreans hoping to defect to the South.

"Closed-circuit television footage shows the soldiers ripped the machine off and ran away. The whole process took just two minutes," another Army source said.

The case prompted the United Nations Command here to send a notice to the North, urging it to stop such provocative actions, and requesting that the South Korean army carry out an extensive inspection to check its readiness against the communist country along the border regions, he noted.

After an incident in October 2012 in which a North Korean soldier knocked on a window of a South Korean military barracks to defect, the authorities here set up dozens of bells in the DMZ to help ensure the safe arrival of possible defectors.

North Korea has employed its signature dual strategy of engagement and provocations in recent months. While firing off short-range missiles in succession and making attempts to infiltrate the border, Pyongyang has made reconciliatory gestures to the South, including making a proposal of suspending all military hostilities and sending a cheering squad to the upcoming Incheon Asian Games in South Korea. (Yonhap)