By Linda Sieg

TOKYO (Reuters) - Japan wants China to help keep North Korea from imploding and might need U.S. defense backup if it does, but its own ability to take diplomatic initiatives is being constrained by its focus on the fate of citizens abducted by Pyongyang decades ago.

Japan, whose territory is within range of its unpredictable neighbor's missile arsenal, has made a show of solidarity with its closest ally, the United States, since Monday's sudden announcement that 69-year-old North Korean leader Kim Jong-il had died.

The question of North Korea's shaky succession will also top the agenda when Prime Minister Yoshihiko Noda makes a previously scheduled visit to China on December 25-26.

But the possible impact of North Korea's leadership change on a search for clues to the fate of the abductees has grabbed much, if not most, of Japan's media attention. Relatives of the kidnapped Japanese say the change provides hope of progress, but admit uncertainty is high.

"You can see how the (abductees')families and supporters are making their case. In this environment, it would be very hard for Japanese politicians and authorities to take proactive moves because they would be criticized," said Narushige Michishita, a professor at the National Institute for Policy Studies.

"Basically, nobody wants to take responsibility for not being proactive or vocal on the abductee issue. It's not about really tackling the issue but avoiding blame."

The plight of the abductees, spirited away from their homeland in the 1970s and 1980s to help train North Korean spies, seized media attention in Japan after Pyongyang admitted in 2002 that its agents had kidnapped 13 Japanese.

Five abductees were repatriated to huge public fanfare in 2002 but Japan wants to know more about eight who Pyongyang says are dead and another four Tokyo believes were also kidnapped.

Nearly a decade after then-prime minister Junichiro Koizumi traveled to Pyongyang for talks with Kim Jong-il that resulted in North Korea's confession, many Japanese are probably less gripped by the topic than in the early, emotional days.

But a highly public campaign by relatives, including the elderly parents of Megumi Yokota -- kidnapped as a 13-year-old schoolgirl in 1977 -- has kept the issue in the limelight and politicians still risk a backlash for any hint that Tokyo's hard stance toward the North might ease.

"For a small group, it's of intense importance and they will make their views known," said Gerry Curtis, a professor at Columbia University in New York.

"What matters are the people who are not indifferent."

LARGELY BYSTANDER

Former prime minister Shinzo Abe, whose popularity owed much to tough talk towards Pyongyang, has been featured in the media since Monday, urging the government to convince the North it must settle the feud if it wants Japanese economic aid.

Japan's stance towards Pyongyang put it at odds with other countries after it refused to provide energy assistance promised to North Korea under a 2007 six-party agreement, under which Pyongyang was to scrap its nuclear program in return for aid.

The countries in the talks are the two Koreas, the United States, China, Japan and Russia.

"Japan was regarded as a kind of impediment to the process," Michishita said. "We shouldn't repeat that this time ... although even that can be difficult."

Japan has also been largely a bystander in efforts to resume those talks, aimed at persuading Pyongyang to give up ambitions to develop nuclear weapons, after they broke down in 2008 and United Nations inspectors were expelled from the North in 2009.

"If there were no abductees issue, Japan would be in a position to be much more supportive of U.S. policies to open up that country by being a major donor in a U.S.-Japan-South Korea effort," Columbia's Curtis said.

Japan this year extended sanctions on North Korea including a ban on trade and travel. The steps were initially imposed after Pyongyang's first nuclear test in 2006 and toughened after North Korea's sinking of a South Korean naval vessel in 2010.

The two countries have long feuded over Tokyo's 1920-1945 colonization of the Korean peninsula, and Japan sees itself as a possible target of Pyongyang's military aggression.

North Korea's 1998 launch of a long-range missile over Japan prompted Tokyo to develop a domestic spy satellite system and beef up its missile defense. Tokyo got another reminder of Pyongyang's reach in 2009 when it test-fired a Taepodong-2 missile that also flew over Japan, splashing down in the Pacific.

(Editing by Paul Tait)