China is lobbying neighbours to sign up to a road map for renewed nuclear disarmament talks with North Korea, whose leader Kim Jong-il is visiting China amid conciliatory words and threats of holy war.

The details of Beijing's plan for restarting stalled six-party nuclear talks came from a South Korean diplomatic source, who spoke on Saturday after discussion in Seoul with Wu Dawei, China's top envoy in the talks.

But the source, as well as a Japanese official speaking in Beijing, stressed that big obstacles remained, even if the secretive Kim's trip to China yields another vow of North Korea's willingness to sit down and discuss a dormant deal to scrap its nuclear weapons in return for aid.

We don't want to restart six-party talks for the sake of talks, the South Korean diplomatic source said. North Korea should change its attitude and show seriousness in denuclearising.

China's regional lobbying, and courting of the reclusive Kim, highlight the pressures that North Korea -- isolated, poor and with a brace of primitive nuclear bombs -- has brought to bear on northeast Asia, home to the world's second and third biggest economies and a big U.S. military presence.

Kim, 68, and his son and heir apparent, Kim Jong-un, were in China to visit the school of senior Kim's father and founder of North Korea, Kim Il-sung, a source with knowledge of the secretive trip told Reuters.

Trust me, it's 100 percent both are here, the source said, declining to give details when asked.

Kim Il-sung attended the Yu Wen High School in the northeastern Chinese city of Jilin in the 1920s. The school houses a memorial hall to Kim which is not open to the public.

The museum was renovated recently ahead of a visit by a group of North Korean dignitaries, a second source said.

Classes were suspended on Thursday amid tight security and a school choir performed for the dignitaries, the second source added, but did not know if the Kims were among the guests.

They sang 'The song of General Kim Il-sung' in Chinese and Korean. It's the school song, the second source said.

There had been no conclusive sightings in China of Kim, who has appeared frail and gaunt since reportedly suffering a stroke in 2008.

Neither source wanted to be identified because of the political sensitivity of the trip. The two neighbours do not disclose much information about Kim's travels, and then only after he has left for home.

DIPLOMAT'S WARNING

On Friday, a North Korean diplomat brandished the possibility of nuclear war with South Korea and the United States.

If Washington and Seoul try to create conflict on the Korean peninsula we respond with a holy war on the basis of our nuclear deterrent forces, North Korea's ambassador to Cuba, Kwon Sung-chol, said in Havana, according to a report from there by China's official Xinhua news agency.

North Korea staged nuclear test blasts in 2006 and 2009, drawing international condemnations and U.N. sanctions backed by China, the biggest economic and diplomatic backer of Pyongyang.

China's envoy, Wu, proposed a three-stage process to restart the multilateral talks aimed at coaxing Pyongyang to give up its nuclear weapons in return for aid and other assurances, the South Korean diplomatic source told Reuters.

Former U.S. President Jimmy Carter returned home from Pyongyang on Friday with an American who had been sentenced to eight years of hard labour for illegally entering North Korea. The North's state media said number two leader, Kim Yong-nam, had told Carter that Pyongyang wanted the nuclear talks resumed.

China has sought to defuse confrontation by hosting six-party nuclear disarmament talks since August 2003. But last April, North Korea quit the talks and reversed disablement steps intended to cripple its chief reactor complex, unhappy with implementation of an initial disarmament agreement reached in 2007.

North Korea has been retreating from its earlier public renunciation of the talks. South Korea and Washington say resuming the talks will be impossible until Pyongyang also faces up to their conclusion that it was behind the sinking of a South Korean navy ship, the Cheonan, in March.

South Korea lost 46 sailors when the Cheonan sank. Seoul said an inquiry found there was no doubt North Korea torpedoed the ship, but Pyongyang denied it was responsible.

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