South Korea's military says its troops fired warning shots when North Korean soldiers briefly crossed the military demarcation line between the two countries on Sunday.
More than 10 North Korean soldiers apparently violated the land border by mistake.
The Joint Chiefs of Staff said on Tuesday that the incident occurred shortly past noon on Sunday.
It added that the North Korean soldiers immediately returned to their territory, after the South Korean military issued warning broadcasts and fired warning shots.
Sources close to the South Korean military said the North Korean soldiers were carrying pickaxes or other tools.
It said the military believes that the North Korean soldiers did not intentionally cross the border.
The move was apparently in retaliation for the anti-Pyongyang balloons that some North Korean defectors in the South sent into the North. »