Reclusive North Korean leader Kim Jong-il reportedly went to China seeking aid and protection from his only major ally after bungled policies at home and military grandstanding that has exasperated the region.
China has propped up the North's leaders for decades. Analysts say even though Beijing is increasingly fed up with its provocative neighbour, it is willing to bankroll Kim to prevent chaos on its border.
Kim, aware of Beijing's predicament, will demand sweeteners to rein in his military and return to international nuclear disarmament talks hosted by Beijing, analysts said.
He crossed into China in the predawn hours in his armoured train and went to the thriving port city of Dalian, Yonhap news agency quoted South Korean officials as saying.
In his last trip in 2006, Kim toured China's industrial centres for a first-hand look under the hood of the country's quickly growing economy.
Dalian, a rebuilt rust-belt city that has attracted major foreign investment, is a symbol of development that Beijing's leaders have advocated for years to Kim and his father, state founder Kim Il-sung, to revive the North's moribund economy.
But Kim has painted himself into a corner.
Economic reforms would open his hermit state and could undermine his "military first" ideology, which justifies economic hardships at home to build a military strong enough to prevent an invasion.
A booking agent at the Furama Hotel in Dalian where Kim was thought to be staying told Reuters it was not accepting reservations for Monday because of "an event".
A highway into Dalian has been blocked to normal traffic and there was a heavy police guard near a factory zone.
There has been no confirmation of the trip, and reporters, camping out along the line in Dandong that Kim's special train would have to use to enter China, were hounded out of the area by Chinese security agents just before the suspected crossing.
The visit would be Kim's first trip abroad since a suspected stroke in 2008. Analysts are also wondering whether Kim's youngest son Jong-un may be joining him so that he could introduce him as the heir to the family throne in Beijing.
Kim's trip comes at an even more precarious time for the North's already struggling economy, hit by U.N. sanctions to punish it for a nuclear test a year ago and a botched currency reform late last year that worsened inflation and sparked almost unheard of civil unrest.
South Korea suspects the North of attacking one of its naval ships in late March, killing 46 of its sailors in what could be one of the deadliest strikes between the rivals since the end of the 1950-53 Korean War.
Fearful a revenge strike could lead to armed conflict that could damage its rapidly recovering economy, Seoul is looking to punish Pyongyang by cutting into its already meagre international finances and sending it deeper into isolation.
This, in turn, could drive Pyongyang even closer to Beijing.
"Beijing has shown great reluctance to forsake pariahs. If anything, it will reinforce the importance of Chinese investment because they will really be the only game in town," said Peter Beck, a Korean affairs researcher at Stanford University.
In 2009, bilateral trade between China and North Korea, with an estimated GDP of $17 billion, was worth $2.7 billion. As the North's economy has grown weaker since Kim took over power in 1994, China has supplied more food, oil and goods that serve as a lifeline for his broken state.
Kim's previous trips to China have led to steps that decreased the security risk the North poses to the region.
Analysts expect a visit to add new life to now dormant international nuclear disarmament talks hosted by Beijing that have been boycotted by Pyongyang for over a year.
The North's official media has not mentioned the trip and did not announce his 2006 visit until after Kim's armoured train crossed the border and he was safely back in North Korea.