The Japanese government decided on a package of additional economic sanctions against North Korea on Wednesday in response to the regime's claim of a nuclear test, including a ban on all imports from the country and the docking of North Korean ships in Japanese ports.
The sanctions are expected to go into effect after they are approved by Japan's Cabinet Friday.
"We will take strong countermeasures," Kyodo quoted Song Il Ho, North Korea's ambassador in charge of diplomatic normalization talks with Japan, as saying in an interview on Wednesday when asked about fresh sanctions by Japan.
"The specific contents will become clear if you keep watching. We never speak empty words," he added.
Kyodo did not explain why the interview, conducted on Wednesday before the sanctions were decided, was not reported until Thursday.
Song said that Pyongyang considered Japan's measures as "more serious in nature" than other nations' because Tokyo has yet to adequately atone for its 1910-1945 colonization of the Korean Peninsula.
"We will be taking countermeasures by calculating that in," Song said.
Song said Pyongyang was closely watching moves by new Japanese Prime Minister Shinzo Abe, who took office last month and is known for his hawkish views on North Korea.
The Kyodo report quoted Song as suggesting that Pyongyang would not hold normalization talks with Tokyo as long as sanctions are in place. Those talks are currently stalled over issues including the abductions of Japanese citizens by agents from the North in the 1970s and 80s.
"I wonder if we can hold talks under these kinds of circumstances," Song said.
Japan prohibited North Korea's ships from entering Japanese ports and imposed a total ban on imports from the impoverished nation.
North Korean nationals also were prohibited from entering Japan, with limited exceptions, the Japanese Cabinet Office said in a statement released after an emergency security meeting late Wednesday.
A total ban on imports and ships could be disastrous for North Korea, whose produce such as clams and mushrooms earns precious foreign currency on the Japanese market. Ferries also serve as a major conduit of communication between the two countries, which have no diplomatic relations.