S.Korean military: N.Korean soldiers cross border again

South Korea's military says its troops fired warning shots after soldiers from North Korea crossed the military demarcation line between the two countries on Tuesday.

Military sources said about 20 to 30 North Korean soldiers breached the line at around 8:30 a.m. but immediately returned to the North after the warning shots were fired.

South Korea's military thinks the soldiers mistakenly crossed the line.

It believes that near the demarcation line, the North is burying landmines, reinforcing roads and building structures thought to be anti-tank defensive barriers. The military released images of the situation on Tuesday.

It believes the soldiers were involved in such operations.

South Korea's military also says it suspects landmines have exploded a number of times during the operations, resulting in numerous casualties.

North Korean troops are thought to be continuing the work, prompting Seoul to conclude they are becoming more active, and to step up vigilance.

A similar incident occurred on June 9. South Korea's military fired warning shots when about 20 soldiers briefly entered the South Korean side.