South Korea sent two helicopters to battle a wildfire inside the Korean Demilitarized Zone (DMZ), Seoul’s military said Friday, adding it had warned North Korea in advance of its firefighting mission along the heavily armed border area.

South Korea fired warning shots after, Seoul says, North Korean troops crossed their border. The soldiers then went back to the North, Seoul said.

As North Korea continues to develop its weapons programs and strengthens its military ties with Russia, a reduction of U.S. troops based in South Korea would be "problematic," the commander of U.S. Forces Korea said.