Kim Jong-il’s second visit to China this year

The reports claimed he was accompanied by his son and possible heir Kim Jong-un.

The apparent visit came amid renewed speculation about a successor to Kim Jong-il, who was 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 the North Korean leader’s second trip to China this year, after a five-day visit in May. The reclusive leader rarely travelled abroad.

Beijing and Pyongyang generally only acknowledged such visits once they were over.

Trade could be a motive for the trip. North Korea relied on aid and trade from China to prop up its tottering state-run economy.

The succession to the ailing Kim could also be on the agenda. He was also reported to have had a stroke two years ago and was thought to be manoeuvring to make his third son, Kim Jong-un, his heir.

There had 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 was discussion of North Korea's nuclear programme.

China had been making moves to resume the six-nation talks on disarming the North, after the sinking of a South Korean warship in March had inflamed tensions between Pyongyang and Seoul. North Korea has been blamed for the sinking.