North Korea is known to have issued its highest level of military readiness in response to the Ulchi Freedom Guardian(UFG) joint exercises between South Korea and the U.S. 

A South Korean military official said Friday that North Korea has typically bolstered its patrols during the UFG period and it's believed the North Korean military has been placed on the highest alert since early this week.

The North issued special security alert No.2 during last year's Ulchi Freedom Guardian joint exercise.

In a statement this past Monday which was the first day of the South Korea-U.S. drills, the General Staff of the North's Korean People's Army issued a threat against the South. 

The statement said that combined striking units of the North Korean military maintain a posture to launch preemptive retaliatory attacks on all enemy forces that are taking part in the Ulchi Freedom Guardian exercise.