South Korea is warning North Korea there will be consequences if Pyongyang carries out a nuclear weapons test.
South Korea has warned that North Korea would "bear entire responsibility for the consequences" of a nuclear test.
The Pyongyang government Tuesday said it would conduct its first nuclear test, but did not say when.
After a security meeting Wednesday, South Korean officials said they would not tolerate a test and urged Pyongyang to return to six-nation talks on ending its nuclear programs.
Yoon Tae-yong is a spokesman for South Korea's president.
Yoon says Seoul has intensified the state of national alert, and is consulting with other countries on the matter.
North Korea says it needs to test a nuclear device to counter U.S. hostility. Washington says it has no intention of attacking the North, and says the impoverished nation can reap economic and diplomatic benefits if it gives up its nuclear arms.
Pyongyang's test plan has been condemned internationally. The United States says a test would threaten regional security.