The United States said that a North Korean nuclear test would be a "deeply provocative act" and urged the world to let Pyongyang know such a test would further isolate the already reclusive nation.
State Department spokesman Sean McCormack called for "all the members of the international community, including China, to state very clearly to North Korea that this would be a very provocative act and it would only add to and deepen their isolation."
South Korea's main spy agency warned last week that Pyongyang could test a nuclear device at any time.
"This is a matter of concern," South Korea's national security adviser, Song Min-soon, said of a potential nuclear test after meeting Wednesday with Secretary of State Condoleezza Rice. "We will remain watchful on any possibilities."