Japanese officials have branded North Korea's failure to provide any new information on 10 missing Japanese nationals as "insufficient".
Japanese media interest in the cases is intense
The two sides have just completed talks in Beijing on the mystery cases.
Pyongyang has previously admitted it kidnapped eight of them, but has claimed they are now dead. It says the other two never entered the country.
But Tokyo is sceptical, and North Korean leader Kim Jong-il pledged in May to reinvestigate the cases.
"We have not received any detailed explanations of the 10 Japanese unaccounted for," a Japanese Foreign Ministry official told reporters.
"We can only regard it as insufficient," Chief Cabinet Secretary Hiroyuki Hosoda told reporters.
The issue is a major stumbling block to the establishment of diplomatic ties.
North Korea admitted in 2002 that it had kidnapped five Japanese men and women, who have since returned home. The admission was seen as part of an attempt by North Korea to normalise relations with Japan and win economic aid.
The kidnap victims were all snatched in the 1970s and 80s and used to help train North Korean spies in Japanese customs and language.
Snatched in the '70s and '80s
Used as cultural trainers for N Korean spies
Five allowed home in 2002
Five children now freed from N Korea
Eight said to be dead, others missing
But Tokyo is not satisfied with Pyongyang's explanation of what happened to 10 other Japanese.
During the two-day talks, which ended on Thursday, North Korean officials did not shed any further light on the matter.
The Japanese delegation also raised the issue of another national who was not on the original official missing list - a man called Susumu Fujita, who disappeared 28 years ago.
Many in Japan believe there are at least scores of other nationals who have been kidnapped by the Stalinist state.
Japanese officials said they had proposed a new round of talks on the issue for next month, but the North Koreans have not yet responded.
Japan's Kyodo news agency - quoting an unnamed Japanese official - said earlier that Tokyo might suspend food aid to North Korea if it fails to provide a satisfactory response on the missing Japanese.
Meanwhile, the American husband of one of the Japanese abductees who has already returned home, Hitomi Soga, is fighting deportation to the US.
Charles Robert Jenkins is currently in a Tokyo hospital, following a reunion with his wife last month.
The former GI, whom the US accuses of defecting to North Korea in 1965, faces a possible court martial.
He has met a military lawyer as a first step to resolving the case against him, Japanese media said last week.