Japan has sternly demanded that North Korea come clean on the fate of missing Japanese it abducted years ago or face retaliation, possibly economic sanctions. North Korea has previously stated that it would consider sanctions tantamount to a declaration of war.
The Japanese government says it has concluded that none of the recent evidence provided by North Korea concerning the abductees is credible.
North Korea claims eight of the Japanese are dead and two others on Tokyo's list never entered North Korea.
Chief Cabinet Secretary Hiroyuki Hosoda on Friday issued one of Japan's most forceful statements yet on the matter.
Mr. Hosoda says Japan strongly demands that North Korea return the survivors.
Without a sincere response, Mr. Hosoda went on, Japan would have to take stern measures, though he did not say what they would be.
Despite Mr. Hosoda's unusually blunt talk, Japanese Prime Minister Junichiro Koizumi says he still hopes dialogue will lead to a breakthrough.
Mr. Koizumi tells reporters he will wait to see how Pyongyang responds before discussing possible sanctions.
Most observers say sanctions would complicate attempts to get the North to return to talks on ending its nuclear weapons programs.
The two Koreas, the United States, Japan, China and Russia have held three rounds of inconclusive talks. A fourth round, set for September, was never held because Pyongyang refused to attend.
Public anger in Japan rose after Tokyo announced that tests showed remains Pyongyang handed over last month were not those of a missing Japanese woman, as North Korea had claimed.
Tokyo on Friday said it would not return the cremated remains to North Korea, despite a request by Pyongyang for them.
North Korea has admitted kidnapping 13 Japanese during the Cold War era to train its spies in Japanese language and culture.
North Korea said the returned remains were those of Megumi Yokota, who was 13 when she was kidnapped from Japan. Pyongyang says she committed suicide at a hospital.
Her father, Shigeru Yokota, on Friday said his government needs to tell Pyongyang time is running out.
Mr. Yokota says Japan has to set a deadline for North Korea to submit evidence about the fate of the missing.
Prime Minister Koizumi on Friday said he does not want to impose such a deadline.