Observers claimed the Pyongyang troops might have mistakenly crossed the boundary while fortifying their side with anti-tank barriers or mines.

In June last year, North Korean troops violated the border three times, prompting South Korea to fire warning shots. The incidents occurred when the Koreas were embroiled in Cold War-style campaigns like balloon launches and propaganda broadcasts, but they didn't develop into a major source of tensions.

South Korea's military fired warning shots on Monday after approximately 10 North Korean soldiers crossed the heavily fortified border that separates the two countries, an incident that underscores rising tensions on the Korean Peninsula amid political instability in Seoul and continued provocations from Pyongyang.