Donald Trump sent a signal to North Korea's Kim Jong-un that the release of Japanese hostages held by the regime – some for more than thirty years – could jump-start talks to ease the crisis.

North Korea has admitted to taking thirteen hostages, releasing five in 2002.  But a Japanese commission set up to investigate the kidnappings put the number at closer to 100, with fifteen cases just since 2000.

CNN:

US President Donald Trump hinted at the possibility of a diplomatic breakthrough with North Korea, were the country to release a number of Japanese citizens abducted by the regime more than two decades ago.

Trump, who is in Tokyo on a diplomatic tour of Asia, made the comments after attending a meeting with the families of those kidnapped by North Korean agents Monday.

Some of the families are still waiting on any information about their loved ones, years after they first disappeared. "I think it would be a tremendous signal if Kim Jong Un would send them back," Trump said during a press conference with Japanese Prime Minister Shinzo Abe following the meeting. "That would be the start of something I think would be just something very special if they would do that."

The issue of North Korean abductions remains highly charged within Japan, where over a dozen people remain missing after being abducted by alleged North Korean agents in the 1970s and 1980s. Pyongyang admitted to some of the abductions in the early 2000s, but Tokyo has accused North Korea of not being completely transparent.

At the press conference, Trump compared the case of one abductee to Otto Warmbier, an American college student who was detained by North Korea and released in dire condition. He died just days after returning to the United States.

"No parent should ever have to endure 40 years of heartbreak. We also had a young wonderful man in our country," Trump said at the news conference. "We all know the story about him. It's a horrible story. It's a sad story and we can't let that happen."

Most of the hostages were kidnapped from Japanese coastal areas.  According to Wikipedia (citations omitted), there are several reasons why North Korea took the Japanese citizens:

In the 1970s, a number of Japanese citizens disappeared from coastal areas in Japan. The people who had disappeared were average Japanese people who were opportunistically abducted by operatives lying in wait. Although North Korean agents were suspected, the opinion that North Korea had nothing to do with the disappearances was widely held. Most of the missing were in their 20s; the youngest, Megumi Yokota, was 13 when she disappeared in November 1977, from the Japanese west coast city of Niigata.

Some of the victims were abducted to teach Japanese language and culture at North Korean spy schools. Older victims were also abducted for the purpose of obtaining their identities, but these abductees are believed to have been killed immediately. It is speculated that Japanese women were abducted to have them become wives to a group of North Korea-based Japanese terrorists belonging to the "Yodo-go" terrorist group after a 1970 Japan Airlines hijacking and that some may have been abducted because they happened to witness activities of North Korean agents in Japan, which may explain Yokota's abduction at such a young age.

For a long time, these abductions were denied by North Korea and its sympathizers (including Chongryon and the Japan Socialist Party) and were often considered a conspiracy theory. Despite pressure from Japanese parent groups, the Japanese government took no action.

There are claims that this issue is now being used by Japanese nationalists, including Prime Minister Shinzō Abe, to "further militarize", push for revision of the Constitution to reduce constitutional limits on the army, revise the Basic Education Law, and pursue other political goals.^[Such claims have been criticized by Kyoko Nakayama, the special adviser in Tokyo to the Japanese prime minister on the abduction issue, who said "This is about rescuing our citizens (from ongoing abduction). They deserve all possible support to regain their freedom and dignity. It is our duty to retrieve them."

Perhaps the most famous North Korean abduction was of South Korean movie star and her husband, who was a famous film director:

Shin Sang-ok and Choi Eun-hee were both snatched in Hong Kong. A similar ruse was used to that tried with the pianist – the lure of meeting in a remote house. The couple spent eight years in North Korea making films there before finally escaping.

The kidnap plot was hatched by Kim Jong-il who, before he succeeded his father as the country's leader, was in charge of its film industry. He was a great film buff, an avid watcher of Hollywood movies – in particular, the first Rambo movie, anything with Elizabeth Taylor and the James Bond films which may have fed his appetite for covert operations.

It sounds incredible, but it's true.  A complete accounting of all Japanese citizens who were kidnapped will probably never be known.  But is there any chance North Korea would acquiesce and release those hostages?

My guess is that most if not all of them are dead.  It was easier for North Korea to kill them rather than have to own up to their being kidnapped.  As the U.S. discovered with Otto Warmbier, North Korea has no hesitancy about mistreating its prisoners, no matter what the crime.  Surviving conditions like that for decades is extremely unlikely.