The government weekly Tongil Sinbo posted an article saying North Korea was involved in projects it declined to identify, but that it would be a mistake to conclude they involved a nuclear weapons test.
"The U.S. and hostile forces jumped to conclusions that the republic is planning the third nuclear test, citing their hypothesis and argument," Tongil Sinbo said.
South Korea's Yonhap news agency said Sunday the article appeared to be a response to the international uproar over North Korea's launch of a long-range missile in December. The North has also announced it would carry out a "nuclear test of a higher level" sometime in the near future.
It remained unclear whether the test would involve an actual nuclear weapon, but Tongil Sinbo warned the United States faced "irrevocable losses" if it responded.