Seoul and Washington issued a joint warning Tuesday, saying North Korea will face "significant action" if it carries out a third nuclear test.
The remarks came after Pyongyang last week threatened to carry out a "higher-level" nuclear test in response to a U.N. Security Council (UNSC) resolution adopted over the North's Dec. 12 long-range rocket launch.
"The statements that North Korea has made have been needlessly provocative, and any test would be a significant violation of UNSC resolutions," U.S. State Department spokeswoman Victoria Nuland said in Washington.
"And as we said in Resolution 2087, we will take significant action if they launch."
In Seoul, foreign ministry spokesman Cho Tai-young backed the remark while urging the North to stand down and pay attention to its impoverished people instead.
The resolution, which adds entities and individuals to a U.N. sanctions list, does not elaborate on further measures.
"I don't really understand why North Korea is sticking to an act that threatens security in the region at a time when its people are struggling from a lack of food," Cho said, without elaborating on possible measures.
Concern is mounting over the possible test as Seoul officials and commercial satellite images suggest the regime could rapidly carry it out after making the political decision to do so. Analysts fear the test could bring Pyongyang closer to its goal of possessing long-range nuclear weapons capability.
Meanwhile the North warned Seoul against moves to implement the resolution, saying it would be an act of "war."
The North would "respond to a war of invasion with an armed struggle to unify the fatherland," Pyongyang's official Korean Central News Agency said in a commentary.
It added that inter-Korean dialogue would be impossible if Seoul kept up its current policy, in an apparent message for the incoming Park Geun-hye administration, which says it wants to "build trust" with the regime.
Last week, visiting U.S. envoy Glyn Davies said his trip to Asia intended to "explore ideas" on how to resume negotiations with the North. Such efforts appear to have been pushed back by the North's escalatory tone and a nuclear test would likely postpone this further.
Such testing is necessary for the North to continue mastering its nuclear weapons technology, including the ability to fashion a warhead small enough to mount atop an intercontinental ballistic missile (ICBM).
The December rocket launch represented a step towards ICBM capability for the North, analysts said.
Rep. Park Jie-won of the main opposition Democratic United Party said Pyongyang could time the test to coincide with the anniversary of the birth of the late dictator Kim Jong-il, or directly ahead of Park's inauguration.
He said the rhetoric was aimed to build internal unity behind leader North Korean leader Kim Jong-un and to send a defiant message to Seoul and Washington.
"North Korea has usually carried out what it repeatedly threatens to do, so I think it will conduct a nuclear test for sure," he said.