A local police source in the Russian region of Chita on China's border said the train carrying the Stalinist state's 69-year-old leader approached the Zabaikalsk-Manchuria border crossing on Thursday evening.

"The train left the (Zabaikalsk) station and headed toward the Chinese border," the unnamed official told the RIA Novosti news agency.

It was not clear whether Kim -- known for shunning air travel and taking extraordinary security measures -- would be stopping in China or heading directly home.

Kim's third trip to Russia in a decade was crowned Wednesday by his first summit meeting with Medvedev at a Siberian garrison near the traditionally Buddhist city of Ulan-Ude.

The talks ended with a Kremlin announcement that North Korea was ready to resume nuclear dialogue without preconditions and abandon atomic enrichment and testing once the talks begin.

But both the United States and South Korea -- who along China and Japan make up the other countries in the so-called six-party nuclear talks -- dismissed the proposal as nothing new.

"If it's true, (it's a) welcome first step, but far from enough ... to resume the six-party talks," US State Department spokeswoman Victoria Nuland told reporters.

South Korean officials demanded that the North abandon its nuclear programme before assuming negotiations on the potential lifting of sanctions and provision of international aid for the impoverished republic.

"I don't see any particular progress," Deputy Spokesman Shin Maeng-Ho of the South's foreign ministry told AFP.

Another Seoul government source told South Korea's Yonhap news agency that the Kim-Medvedev meeting "fell short of expectations".

Moscow's Kommersant business daily noted simply that Medvedev "failed to reach a global breakthrough on North Korea."

Moscow's influence over Pyongyang has waned considerably since end of the Soviet era and Kim's visits to China have become much more frequent than the ones to Russia -- a country he last travelled to in 2002.

Kim completed his third trip to China since the start of last year in May and will be travelling through the country again at the same time as it hosts South Korean nuclear negotiator Wi Sung-Lac.

North Korea's KCNA state news agency did not mention China in a Wednesday evening dispatch announcing "the conclusion of (Kim's) historical visit to the Russian Federation."

Medvedev said he was "full of positive feelings" after meeting Kim and optimistic that the two sides were making progress on the construction of a key pipeline through North Korea to the South.

"As far as I understand, North Korea is interested in the implementation of such a trilateral project with the participation of Russia and South Korea," Medvedev said.

The gas link would provide Russia with broader access to the booming South Korean energy market and have the potential of uniting the two peninsula neighbours behind a single project at a time of flaring tensions.

But some in Russia questioned whether the pipeline was feasible considering the level of global distrust of the North Korean regime.

"From the standpoint of (gas) supply security, this project is madness for Russia at the moment," the Vedomosti business daily wrote in an editorial.

The idea that the project should help build trust between the two Koreas "is beautiful but very risky and requires serious sponsorship from the Russian people," the paper said.