A group of armed North Korean soldiers crossed the military border into the South and returned to the North instantly after receiving warning shots, a South Korean Army source said Sunday.

"The military sent a warning message and fired warning shots after some 10 North Korean soldiers crossed the Military Demarcation Line (MDL) near Cheorwon, Gangwon Province, at about 8 to 9 a.m. on Saturday," the source said. "The North Korean soldiers were armed and retreated after the warning shots. They didn't fire back", South Korean News Agency (Yonhap) reported.

This marked the first time this year that the South Korean military has fired warning shots at North Korean soldiers for intruding.

In October, soldiers of the two Koreas exchanged fire across the heavily armed border as North Koreans responded to the South's warning shots when they approached the MDL in Paju, Gyeonggi Province. There were no reports of casualties or property damage on both sides.

It was not immediately known what prompted the North Korean soldiers to try to cross the border in Cheorwon, a city located 88 kilometers north of Seoul.