North Korean leader Kim Jong Il waves goodbye to Chinese leaders at a train station in Beijing, China, after a four-day visit in May 2010. According to reports, Kim returned to China on Thursday for his second visit of the year. (Korean Central News Agency/Associated Press)

North Korea's reclusive leader Kim Jong Il was in China Thursday on his second visit this year to his country's biggest source of diplomatic and financial support, according to teachers at a school he visited.

The visit, which has not been announced by either country, is highly unusual, coming just three months after the last visit of Kim, who rarely travels.

But his stop in Jilin city in Jilin province in northeastern China was confirmed by two teachers at the Yuwen Middle School.

"He definitely came over. But I'm not sure if his son was with him or what time he came," said a physical education teacher who would give only his surname Zhao. Another teacher said Kim visited the school in the morning for about 20 minutes.

Kim may be travelling with a son to consult with Chinese officials on plans to transfer power to a successor, South Korea's Yonhap News Agency and YTN television in Seoul said.

It's widely believed that Kim is preparing to transfer power to his third and youngest son, Kim Jong Un, and many North Korea watchers believe the son will be granted a key party position next month.

Citing an unidentified high-ranking Seoul official, Yonhap said Kim's special armoured train crossed the border into China early Thursday. YTN carried a similar report but said Kim arrived in China late Wednesday night.

Succession speculation

The surprise trip comes as former U.S. president Jimmy Carter makes a rare visit to North Korea on a private mission to secure the freedom of an imprisoned American. There was no word on whether Carter — who met in 1994 with Kim Il Sung — had been scheduled to meet the current leader.

There was no comment from China's Foreign Ministry or from the Communist Party's international liaison department, which deals with relations with North Korea.

North Korea announced in June that new ruling Workers' Party leaders would be elected in early September, sparking speculation that the move is aimed at boosting a government campaign to hand over power to a Kim heir.

Speculation on the succession intensified after Kim, 68, reportedly suffered a stroke in 2008. Kim's health problems prompted concerns about instability and a possible power struggle in the nuclear-armed country if he were to die without anointing a successor.

Kim last visited China in May, meeting top leaders, including President Hu Jintao.