The U.S. military is holding exercises in South Korea amid increased tensions with North Korea, whose leader Kim Jong Un has ordered his country’s nuclear weapons to be at the ready. About 2,100 Marines and sailors recently arrived in South Korea for exercise Ssang Yong 16, which began on Wednesday and lasts until March 20, Marine Corps officials said.

Held every two years, the exercise involves U.S. and South Korean troops conducting amphibious operations for possible disaster relief or wartime missions, said 2nd Lt. Joshua Hays, a Marine Corps spokesman. Meanwhile, North Korea has recently tested a nuclear weapon and fired six projectiles into the sea, said Bruce Klingner, a Korea expert at the Heritage Foundation think tank in Washington, D.C. READ MORE