South Korean soldiers fired warning shots to repel North Korean soldiers who temporarily crossed the rivals' land border on Tuesday for the second time this month, South Korea's military said.

The South's military observed increased North Korean construction activities along their heavily armed border to install suspected anti-tank barriers, reinforce roads and plant land mines. The work has gone on uninterrupted despite several explosions caused by mines that killed or injured an unspecified number of North Korean soldiers, said the South's Joint Chiefs of Staff.