South Korea's top military officer called on troops Thursday to prepare for possible "surprise" enemy provocations as he visited a key Air Force unit participating in a major annual exercise with the United States this week, the Joint Chiefs of Staff said.

JCS Chairman Gen. Kim Seung-kyum made the call as he inspected the Korea Air and Space Operations Center at Osan Air Base in Pyeongtaek, 60 kilometers south of Seoul, amid the ongoing Vigilant Defense drills, which kicked off a five-day run Monday.

"We must thoroughly prepare for the enemy's surprise and reckless provocations," Kim was quoted as saying. "In case of an enemy provocation, we must acquire capabilities and a posture for immediate deployment to strongly punish the origin of provocation and other support forces."

Kim also visited the US 7th Air Force at the same air base and was briefed on the combined exercise, which mobilizes some 130 warplanes from the two sides, including variants of the F-35 stealth fighter jet.

Pyongyang has long accused the allies' joint military drills as rehearsals for an invasion against it.

North Korea staged a flurry of missile launches during last year's exercise, named Vigilant Storm, in protest. (Yonhap)