North Korean leader Kim Jong-il is reported to have met Chinese President Hu Jintao for talks in Beijing.
China has hosted six-nation talks on the continuing nuclear stand-off
South Korean media reports said a special train took the reclusive Mr Kim and his entourage to the Chinese capital for a summit on Monday.
China is a key mediator in the row over North Korea's nuclear programme.
Talks have made little progress on how the programme could be dismantled, or how North Korea's energy and security concerns would be addressed.
If confirmed, the North Korean leader's four-day visit to Pyongyang's old communist ally would be his first since May 2001.
No mention of the visit was made on North Korea's official KCNA news agency or on China's official Xinhua news agency, but it was reported extensively in the South Korean media.
Witnesses reported seeing a motorcade leaving the train station in Beijing.
South Korea's Yonhap news agency said talks between Mr Kim and Mr Hu focused on North Korea's nuclear ambitions and Beijing's economic assistance to the North.
Mr Kim was also expected to meet other Chinese leaders, including former President Jiang Zemin and Prime Minister Wen Jiabao, Yonhap said.
Mr Kim, who is scared of flying, likes to travel by special train
Mr Kim's special train crossed into China late on Sunday and travelled overnight to Beijing, South Korea's YTN cable television news said. The North Korean leader is scared of flying.
"At the moment all parties are working together to find a way to set up a working group to address issues," Chinese Foreign Minister Li Zhaoxing told a news conference without confirming Mr Kim's visit.
"China's objective is clear. The legitimate security concerns of [North Korea] should be properly addressed and the whole peninsula should be free of nuclear arms," he said.
Mr Kim's two previous known trips to China since 2000 were confirmed by the two governments only after he returned home.
Analysts say Pyongyang may be seeking economic and energy aid from its old communist ally.
But Beijing is under pressure from Washington to step up the pace of diplomacy over North Korea's nuclear weapons programme.
During his visit to Asia last week, US Vice-President Dick Cheney told Chinese leaders that time was running out to resolve this issue.
He reportedly presented Beijing with new evidence regarding North Korea's nuclear capability.
A North Korean foreign ministry spokesman responded on Sunday by describing Mr Cheney as "mentally deranged".
He said North Korea was "seriously contemplating a measure" to counter the US' demand that Pyongyang irreversibly dismantles its nuclear programme before it will grant the North any concessions.
China has proved a crucial player in dialogue with Pyongyang, having hosted two rounds of six-nation talks in Beijing.
In February, China said North Korea had agreed to push towards a third round of international talks on the region's nuclear crisis.
The nuclear crisis was sparked in October 2002 when US officials said North Korea had admitted to having a secret uranium-based nuclear programme, in violation of a 1994 agreement.
It has since restarted a mothballed nuclear power station, thrown out United Nations nuclear inspectors and pulled out of the Nuclear Non-Proliferation Treaty.
North Korea says it has reprocessed thousands of spent nuclear fuel rods at the Yongbyon nuclear facility, from which extracted plutonium can be used to manufacture nuclear bombs.
The US insists that Pyongyang must dismantle its nuclear facilities. But Pyongyang says it will only do so in return for economic and energy aid, and security guarantees from Washington.