The Joint Chiefs of Staff says it put the military on heightened alert just hours after North Korea threatened an "all-out confrontational posture" against South Korea.

This is the first heightened alert by Seoul against Pyongyang since October 2006, when the North announced conducting a nuclear test.

A Joint Chiefs of Staff official said Sunday that an intensified guard by the Army, Navy and Air Force was ordered at 6 p.m. Saturday, urging increased reconnaissance flights by spy planes.

He also warned of armed clashes that might take place in disputed waters in the Yellow Sea.

A spokesman for the North's Korean People's Army said Saturday that since South Korea has opted for confrontation, the North’s "revolutionary armed forces are compelled to take an all-out confrontational posture to shatter them."