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.

South Korean soldiers fired warning shots at North Korean troops after nearly a dozen crossed the demilitarised zone separating the warring nations.

South Korea's military said Tuesday it fired warning shots at a group of North Korean soldiers who briefly crossed the military demarcation line inside the Demilitarized Zone dividing the two Koreas.