South Korea has increased vigilance against additional provocations by North Korea in the wake of its rocket launches seen as a show of protest against ongoing joint military drills between Seoul and Washington, South Korea (Yonhap) news agency reported Monday. In the latest in a recent series of launches, Pyongyang fired 25 short-range rockets into waters off its east coast Sunday evening, which flew about 70 km. The South Korean military judged the rockets to be FROG short-range surface-to-surface missiles, which the North introduced in the 1960s. "The military is closely monitoring the (eastern) region, as North Korea could additionally launch (rockets)," Defense Ministry spokesman Kim Min-seok was quoted as saying at a press conference in Seoul. Sunday's rocket launch came two days after the North's National Defense Commission threatened to demonstrate its nuclear deterrence capability against the US hostility. Kim said the trajectory of Sunday's rockets indicated the rockets are FROG missiles, adding the South Korean military had detected its moves ahead of the launch. "The military is currently looking into its intention but currently considers (the rocket launch) as a provocative act in a show of force." Kim also condemned Pyongyang's firing of rockets without declaring a no-fly, no-sail zone, potentially endangering civilian vessels or flights in their trajectory. The spokesman added that the North was already ready to conduct a fourth nuclear test, though it has shown no imminent sign.