On Wednesday one source in Beijing had said Kim was on his way to Russia.
But South Korea's Yonhap news agency reported that the North Korean leader was in Shanghai - China's main showcase for the kind of market-oriented economic reforms he has so far just tinkered with - and was due to leave later in the day.
Russian and Chinese Foreign Ministry officials declined to comment on his whereabouts, and South Korean officials said they were struggling to confirm various reports.
Media reports and diplomats say Kim - like his late father, state founder Kim Il-sung - has an aversion to flying and has almost always travelled by train under tight secrecy on his rare visits abroad.
His 2004 trip to China, Pyongyang's main ally and source of aid, was not officially confirmed until he had returned home.
His trip to Moscow in 2001 involved 24 days of travelling as he dined on delicacies and followed news using special communications.
It was not clear what had prompted Kim to travel now.
China is one of North Korea's
few remaining allies
If he is heading for Russia, he could be seeking again to balance the North's relations between Beijing and Russia, which under President Vladimir Putin has been friendlier towards the North than it was during the immediate post-communist years.
If Kim is stopping in China, diplomats and North Korea experts say he is likely to be studying economic reforms, seeking more aid and also support amid his standoff with Washington over his nuclear deterrent and US sanctions.
The Korea Times newspaper said a China trip would most probably be linked to US financial sanctions.
Washington has cracked down on firms suspected of involvement in counterfeiting, money laundering and the drug trade by the North, which it says funds the North's nuclear programs.
"Which side China will take is a critical factor for Kim," the Korea Times quoted Baek Seung-joo of the Korea Institute for Defence Analyses as saying.
Whether to Russia or China, Kim's trip would come as regional powers seek to nudge the North back into talks on its nuclear ambitions.
Chinese President Hu Jintao visited North Korea in October on a trip that was seen as underscoring Beijing's role in persuading Pyongyang to abandon its nuclear weapons programs.