Chinese television footage of Kim meeting Hu and other Chinese officials showed the North Korean leader speaking animatedly and gesturing with both hands.

Kim wore thick glasses, his trademark grey-green suit, and had thinning hair. The swelling that was obvious in his left hand after an apparent stroke in 2008 was no longer apparent.

Japanese scepticism

Japan said that it welcomed the focus on the six-party disarmament talks during the visit to China, but was sceptical that there would be any tangible progress on the issue.

"I think it was good that there was mention of the six-party talks," Katsuya Okada, Japan's foreign minister, told a news conference.

"But realistically speaking, I don't think the talks can move forward unless the issue of the South Korea ship becomes clear."

Tensions between North Korea and its southern neighbour have been heightened since a South Korean naval vessel was sunk in mysterious circumstances close to the disputed maritime border between the two neighbours.

Traces of explosives similar to the type found in torpedoes have been found on the wreckage of the Cheonan, which went down after an explosion killing 46 people, the Yonhap news agency reported on Friday.

But Seoul has not formally accused Pyongyang of being involved, with Won Tae-Jae, the defence minister, saying "no conclusion has been made".

North Korea has repeatedly denied that any the sinking was caused by one of its torpedoes or a mine.

Korean ally

The issue is likely to dominate a series of talks this month between China, Japan and South Korea, first among foreign ministers and then their leaders.

China is the North's only real ally, providing the impoverished nation with much needed support and trade.

During Kim's recent visit, he toured the northern Chinese port cities of Dalian and Tianjin, both models for Chinese economic and investment policies.

The North Korean leader also said he welcomed Chinese investment in his country "based on the principles of mutual benefit and win-win", the Xinhua report said.

Witnesses at the Chinese border city of Dandong said Kim's armoured train had now crossed the border back into North Korea.

Kim has visited China four times since 2000, each time by train and each time under intense secrecy.