The comments were attributed to a senior diplomat, Song Il-ho, on Thursday by Japan's Kyodo news agency.
"We will take strong countermeasures," he was quoted as saying in a statement from Pyongyang. "The specific contents will become clear if you keep watching. We never speak empty words."
Song is North Korea's ambassador in charge of diplomatic normalisation talks with Japan and was referring to Tokyo's decision to implement new steps in response to Pyongyang's announcement on Monday that it had conducted a nuclear test.
The additional sanctions proposed by Japan on Wednesday include banning North Korean imports and blocking ships from entering Japanese ports.
The steps will be approved by the cabinet on Friday, and Japanese officials said Tokyo would consider further measures depending on North Korea's future behaviour and developments in the international community.
Japanese Prime Minister Shinzo Abe said on Wednesday that Tokyo had decided on its own measures in view of the fact that a North Korean nuclear weapons programme, coupled with its missile arsenal, would pose a grave threat to Japan.
Song was quoted as saying that Pyongyang would regard Japan's measures as "more serious in nature" compared to others, because Tokyo has yet to adequately atone for its colonisation of the Korean peninsula from 1910 to 1945.
The US is seeking a United Nations Security Council resolution by Friday that would allow for punitive measures against North Korea.