North Korea is demanding Japan to drop sanctions that Tokyo imposed after Pyongyang launched a series of missiles this week.
South Korea has responded to the missile tests by postponing inter-Korean military talks and withholding food aid - but says senior-level talks on economic cooperation scheduled for next week will move ahead as planned.
Song Il Ho represents North Korea in discussing normalization of diplomatic relations with Japan, but his words to Japanese journalists in Pyongyang Friday were anything but diplomatic.
He warned Japan to withdraw penalties against his country, imposed after North Korea's launch of missiles earlier this week, or face serious consequences.
Song says if there are sanctions, there will be a disastrous result, and Japan will be responsible.
Following the outcry over its missile tests, North Korea warned the world that it would take "stronger physical actions" against any nation that dared to "take issue with the exercises" and put pressure on Pyongyang.
But Japan quickly imposed restrictions on North Koreans traveling to Japan, and barred a North Korean ferry for six months from making port calls in Japan.
Japanese leaders say stronger sanctions may be on the way. Japan's chief cabinet secretary, Shinzo Abe, replied by saying Song's comments were "extremely regrettable," and that Japan felt "resentment."
South Korean officials, meanwhile, said Friday they had declined for now Pyongyang's proposal for working-level military talks with the North.
Seoul says the timing is not appropriate following the missile tests, which included a long-range ballistic missile potentially capable of reaching the United States.
South Korean officials also say they are reviewing key aspects of North-South cooperation, such as food aid, and late Friday, local media reports quoted an official as saying food shipments would be delayed indefinitely.
However, South Korean Foreign Ministry spokesman Lee Guan-se says Seoul intends to push forward with its policy of dialogue with Pyongyang.
He says inter-Korean ministerial talks will take place as scheduled next week in the South Korean city of Busan.
He says South Korean officials will discuss the missile launches, as well as urge North Korea to return to six-nation talks aimed at ending its nuclear weapons capabilities.
North Korea has refused to return to those talks, despite a pledge it made in September to Japan, Russia, China, South Korea and the United States to begin the process of dismantling its nuclear programs.
Pyongyang's Foreign Ministry says its missile launches are a matter of national defense, and are completely separate from the nuclear negotiations.
Christopher Hill, the U.S. assistant secretary of state who is Washington's chief envoy to the nuclear talks, is expected to arrive in Seoul Friday to discuss the North Korean situation.
Hill has been in Beijing, where he had similar discussions with senior Chinese leaders, and he is due to fly on to Tokyo on Saturday to continue the discussions there.
