(Reuters) - Rebels in Sudan's oil-producing southern border state of South Kordofan have launched an assault on a town near the border with newly independent South Sudan, army and rebel spokesmen said on Friday.  The poorly marked border region with South Sudan, which gained independence last July, has seen clashes between Sudan and rebels of the Sudan People's Liberation Movement-North (SPLM-North) as well as between Sudan and South Sudan, which fought for two days earlier this week.