"Security is a temporary condition," Brooks, the commander of the US Forces Korea, told a defense forum in Seoul.
"If you are not attending to your security and continuing to advance your capabilities, then you'll be in danger and your security will begin to move away. We never want to see that happen in the Republic of Korea, never again will the Republic of Korea be insecure," he added.
Earlier in the day, the allies announced the much-anticipated decision to suspend "all planning" for the Ulchi Freedom Guardian exercise amid dialogue efforts aimed at denuclearizing North Korea.
Shortly after his Singapore summit with North Korean leader Kim Jong-un last Tuesday, US President Donald Trump unveiled his plan to stop "provocative, inappropriate and expensive" war games with the South, which Pyongyang has decried as a rehearsal for invasion.
The decision to suspend the UFG has emerged as a subject of heated debate here.
Supporters called it a good-will gesture to encourage the North to denuclearize, while naysayers argued that it would undermine the allies' readiness posture and erode the long-standing security alliance.
The UFG is a command post exercise based on computer simulated war games centering on a scenario of an all-out war with the North. In 1990, the allies temporarily halted the UFL due to US participation in the Gulf War.
In 1954, the US-led U.N. Command started the Focus Lens exercise. It was later combined with South Korea's Ulchi exercise, which was launched in the wake of a North Korean infiltration in 1968. The combined exercise, which was called the Ulchi Focus Lens, was renamed the UFG in 2008. (Yonhap)