South Korean officials are playing down security fears on the peninsula amid what seem to be daily threats from the North and a warning that a nuclear war was imminent.
A Defense Ministry official in Seoul says South Korea has deployed three naval destroyers, an early warning surveillance aircraft and a land-based radar system.
But many North Koreans are focused today on the first anniversary of their leader's appointment as head of the ruling Worker's Party.
Still, at the border: North Korea remains tense as more South Korean workers return.
South Korea's presidential security adviser Chun Yung-woo says Tuesday's nuclear test has been confirmed and that it cannot be tolerated. He provided no details on how Seoul confirmed the test.
A nuclear test could take North Korea closer to its goal of building a nuclear warhead that can be mounted on a long-range missile.