China has formally invited Kim Jong-un, the youngest son of North Korea's leader Kim Jong-il and his likely heir, to visit, South Korean sources say.

The information was relayed to South Korean lawmakers in a security agency briefing and has not been confirmed by Beijing.
No timetable was known for the visit, the security sources said.
The agency also said the North was tightening security amid ongoing unrest in the Middle East.
The briefing was relayed to South Korean lawmakers on a parliamentary intelligence committee by the National Intelligence Service, South Korean media said.
Lawmakers were told the invitation to visit was made during recent visits to North Korea by Chinese State Councilor Dai Bingguo and Minister of Public Security Meng Jianzhu.
The sources said it was likely the invitation would be accepted.
""But it remains unclear whether Kim Jong-un will visit China alone or simply accompany the leader,"" the sources said.
China remains North Korea's only major ally.
Kim Jong-un was named to a senior party post and promoted to four-star general last September, signaling the move towards a transition from father to son.
Kim Jong-il is in poor health after a reported stroke in 2008.
Tension on the Korean peninsula soared last year with the sinking of a South Korean warship and the shelling of a Southern island.
The briefing also referred to increased security in the North.
""We understand the deployment of tanks was made for fear of concerns over the fallout from the pro-democracy struggles in the Middle East,"" it said.
(Source: BBC)