"Sanctions are nonsense. If full-scale sanctions take place, we will regard it as a declaration of war," the Beijin-based official, who wasn't identified, told Yonhap.
The statement is a repetition of the North's long-held stance on sanctions.
Separately, a North Korean diplomat said that the country's announced nuclear test was smaller in blast force than expected, though asserted Pyongyang was able to detonate a more powerful device, a South Korean newspaper reported Wednesday.
The Hankyoreh daily didn't name the diplomat at the North Korean Embassy in Beijing but quoted him as saying the test was a success and "smaller in scale than expected."
"But the success in a small-scale (test) means a large-scale (test) is also possible," he said.
The diplomat also said the North could take "additional measures" such as putting a nuclear warhead on a missile, and that the communist nation doesn't fear sanctions.
"The US should show its dialogue attitude in action if it wants to solve the problem," he said in a telephone interview Tuesday, according to the paper. "Otherwise, we continue to go on this footing. We have been under sanctions so far, and therefore there won't be greater hardship for us."
'Resolve situation before unfortunate incident'
Another North Korean official warned the country could fire a nuclear-tipped missile if the US spurns dialogue on Pyongyang's terms in the wake of the country's atomic test, Yonhap news agency reported Tuesday.
"We hope the situation will be resolved before an unfortunate incident of us firing a nuclear missile comes," an unnamed North Korean official said Monday, according to a Yonhap report from Beijing. "That depends on how the US will act."
Even if the North is confirmed to have a functioning atomic bomb, most experts don't believe it has a design small and light enough to place on a missile. Their long-range missile capability also remains in question, after a test rocket in July apparently fizzled shortly after takeoff.
The North Korean official who spoke with Yonhap also indicated that the North wants direct talks with the US, saying that the test is an "expression of our intention to face the United States across the negotiating table."
The official also dismissed moves at the UN Security Council to sanction the impoverished nation over its reported nuclear test.
Monday's test strained the North's relations with its main ally China, with Beijing swiftly denouncing Pyongyang. South Korea's nuclear envoy also said Tuesday, after a trip to China, that Beijing appeared to be leaning toward backing sanctions against the North. But the North Korean official told Yonhap that China "won't give up on us after all."