North Korean leader Kim Jong-il has left for a visit to China, South Korean media are reporting.
Yonhap news agency quoted a South Korean official saying there were "signs" that Kim Jong-il was using his special train to make the visit.
The reports said he was accompanied by his son and possible heir Kim Jong-un.
The apparent visit comes amid renewed speculation about a successor to Kim Jong-il, who is believed to have suffered a stroke two years ago.
"Signs have been detected Chairman Kim made the trip to China early Thursday," a South Korean official told Yonhap. "We are now in the process of finding out specific destinations and the purpose," of the trip.
If confirmed, it would be Mr Kim's second trip to China this year, after a five-day visit in May. The reclusive leader rarely travels abroad.
Beijing and Pyongyang generally only acknowledge such visits once they are over.
Trade could be a motive for the trip, says the BBC's John Sudworth in South Korea's capital, Seoul. North Korea relies on aid and trade from China to prop up its tottering state-run economy.
Or the succession to the ailing Mr Kim could be on the agenda, says our correspondent.
Mr Kim is reported to have had a stroke two years ago and is thought to be manoeuvring to make his third son, Kim Jong-un, his heir.
There have been widespread reports of a major conference of the ruling party of North Korea being held soon to discuss this issue.
One South Korean TV station cited a South Korean official as saying Kim Jong-un was with his father on the trip.
Another possible motive for the trip, say analysts, is discussion of North Korea's nuclear programme.
China has been making moves to resume the six-nation talks on disarming the North, after the sinking of a South Korean warship in March inflamed tensions between Pyongyang and Seoul.
North Korea has been blamed for the sinking.
Meanwhile, former US President Jimmy Carter has spent the night in Pyongyang trying to secure the release of a US citizen detained by North Korea since January.
Aijalon Mahli Gomes was sentenced to eight years' hard labour for illegally entering North Korea.
There are reports that he could leave with Mr Carter some time on Thursday.