South Korea’s military on Friday said it had fired warnings shots the previous day to repel North Korean soldiers who temporarily crossed the demarcation line between the two countries for the third time this month.

South Korea’s Joint Chiefs of Staff said several North Korean soldiers while engaging in unspecified construction work on the northern side of the border briefly intruded the military demarcation line that bisects the countries at around 11am on Thursday.

The South Korean military in response broadcast a warning and fired warning shots, to which the North Korean soldiers retreated.

The joint chiefs didn't immediately release more details.

South Korea’s military has said it believes the previous two border intrusions this month weren’t intentional as the North Korean soldiers retreated after the warning shots and the North did not return fire.

The South’s military said it had been observing increased North Korean construction activity in front-line border areas since April, such as installing suspected anti-tank barriers, reinforcing roads and planting land mines. (AP)