North Korea's reclusive leader, Kim Jong-il, has reportedly been touring hi-tech firms in Shenzhen, the city at the forefront of China's economic boom.
Beijing usually reports the visit after Mr Kim has left the country
Japanese TV has broadcast secretly filmed footage showing a man resembling Mr Kim leaving a hotel in Shenzhen.
The visit has prompted speculation that China is advocating market reforms for Pyongyang and could also urge it to rejoin stalled nuclear negotiations.
China has not confirmed or denied it is hosting the North Korean leader.
Mr Kim's previous visits to China have only been reported by Beijing after his return home.
The leader travelled to China in an armoured train last week, according to South Korea's Yonhap news agency.
However, a North Korean military official interviewed by Russia's Itar-Tass news agency says Mr Kim has not left the country.
The footage broadcast by Japanese television shows a man with Mr Kim's dark glasses and bouffant hair leaving a hotel in Shenzhen.
The communist leader spent Sunday visiting high-tech companies in the economic boom town near the border with Hong Kong, according to Japan's Kyodo news agency.
He also reportedly visited the Yantian container port, one of the most advanced in China.
On Saturday, Mr Kim is said to have toured high-tech agricultural facilities.
Mr Kim's visit to the cradle of the Chinese boom could be a careful hint to the outside world that he is following Beijing's lead, says the BBC's Louisa Lim in Beijing.
He may be trying to indicate he is ready to introduce reforms to relieve North Korea's struggling economy, she says.
Our correspondent says Mr Kim is unlikely to leave China without meeting its leaders, who will urge him to return to stalled six-party talks on nuclear disarmament.
North Korea agreed at six-party talks in Beijing in September to abandon a nuclear programme in return for economic and security guarantees.
But soon after, it said it would not scrap its nuclear deterrent until it was given a civilian nuclear reactor.
Both Japan and the US have rejected Pyongyang's demand for a reactor.
China, which has hosted the talks and is seen as the only country with influence over North Korea, is keen for the talks to succeed.
Kim Jong-il, who has ruled the Stalinist regime since the death of his father Kim Il-sung in 1994, rarely travels abroad. When he does so he refuses to fly, instead travelling by a special armoured train.