North Korea's foreign ministry says the country will conduct a nuclear test to bolster its defenses against what it calls US hostility.
In response to that announcement Tuesday, South Korea said it has strengthened its security level and alert status.
The Japanese and British foreign ministries condemned the announcement, saying such a test would result in serious consequences.
But the Russian foreign ministry cast doubts on the threat, saying North Korea has made similar claims before.
Moscow said those claims have not been substantiated. The official Korean Central News Agency says the nuclear test will take place "in the future."
It did not specify a date. North Korea has long maintained that it has a nuclear arsenal, but observers say no known tests have been conducted.
Pyongyang says U.S. pressure has "compelled" North Korea to test one of its nuclear devices.
In its statement, North Korea says it would never use nuclear weapons first, and it called for a Korean peninsula free of nuclear weapons.
North Korea says financial sanctions Washington has imposed on North Korea are adversely affecting the country's security.
Information for this report is provided by AP and REUTERS.