Pyongyang didn't say Thursday when exactly it would carry out a third test, but it could happen "at any time," according to Pinkston.

He said it is hard for anybody outside the North Korean nuclear sector to know whether the country is technically ready to carry out the test, but that politically, "it seems an appropriate time."

Demands unlikely to sway North

South Korean defense officials said last year that they believed the North was in a position to carry out a new test whenever leaders in Pyongyang gave the green light.

North Korea's nuclear program is "an element of threat to peace not only for Northeast Asia but also for the world," Park Soo-jin, deputy spokeswoman for the South Korean Unification Ministry, said Thursday.

"North Korea should immediately stop its nuclear test and other provocation and should choose a different path by cooperating with the international community," Park said.

That appears unlikely at this stage, though.

After a failed long-range rocket launch in April, North Korea ignored international condemnation and carried out a second attempt last month. That one succeeded in putting a satellite in orbit, Pyongyang's stated objective.

But the launch was widely considered to be a test of ballistic missile technology. And it remains unclear whether the satellite, which the North insists is for peaceful purposes, is functional.

Both North Korea's previous nuclear tests took place weeks or months after long-range rocket launches.

Those tests were carried out under the rule Kim Jong Il, the deceased father of the current leader, and the man who channeled huge amounts of money into North Korea's nuclear and missile development programs.

Kim Jong Il died in December 2011 after 17 years in power, during which the North Korean people became increasingly impoverished and malnourished.

Economically, the country relies heavily on trade with its major ally, China.

China, which voted in favor of the U.N. Security Council resolution this week, appealed for calm on Thursday.

Foreign Ministry Spokesman Hong Lei urged North Korea and the West to "keep calm, remain cautious and refrain from any action that might escalate the situation in the region."

The office of the South Korean president-elect, Park Geun-hye, said Friday that she would soon send a delegation to visit the United States.

"Issues of common interest will be discussed," said Yoon Chang-jung, a spokesman for Park, without specifying whether that included North Korea.