Sudan rebels trying to move front closer to Khartoum
* Sudan rebels trying to move front closer to Khartoum
* Rebel alliance briefly occupied central city
* Other fighting reported near Nile port Kosti (Edits, adds detail)
KHARTOUM, May 10 (Reuters) - Up to 40,000 people have fled since rebels launched a major assault in central Sudan, the United Nations said on Friday, amid signs of a new insurgent campaign to push closer to the capital.
The Sudanese Revolutionary Front (SRF), an alliance of four rebel groups from the western Darfur region and zones in the south, attacked the central city of Um Rawaba, a usually placid commercial hub, and other areas two weeks ago.
The SRF's brief occupation of the city in North Kordofan state was the closest insurgents bent on toppling President Omar Hassan al-Bashir have got to Khartoum since one rebel band launched an unprecedented raid on its suburb Omdurman in 2008.
Sudan has been plagued for decades by clashes between the government and rebels from its peripheries, fighting against what they say is exploitation by a Khartoum elite.
Simmering rebellions have frustrated efforts to bring stability to a country in one of Africa's most unstable neighbourhoods.
Over the past two weeks, both rebels and government forces have also reported clashes in the northeastern tip of South Kordofan, an oil-producing state, in areas near Kosti, Sudan's biggest Nile river port, and key sugar plants.
The latest battle was reported on Sunday.
Almost 40,000 people had fled clashes in different areas of South Kordofan, and most of them had taken refuge in the town of El Rahad, the United Nations said in a report.
"HAC (the Sudan government's Humanitarian Aid Commission) expects more people to arrive in El Rahad in the coming days," the U.N. Office for the Coordination of Humanitarian Affairs said.
Sudanese authorities said only a few families were displaced by the original attack on Um Rawaba, 500km (300 miles) south of Khartoum.
The army regained control of the city the same day but the coordinated attack prompted fears the insurgents could resume their push on the capital.
The SRF is made up of three rebel groups from Darfur, scene of a decade-long insurgency, and the Sudan People's Liberation Movement-North (SPLM-N), fighting along the border with South Sudan.
SPLM-N is made up of fighters who sided with south Sudan during decades of civil war that ended in a 2005 peace deal.
When South Sudan seceded in 2011 the SPLM-N fighters were left on the Sudanese side of the border, and continued to complain of mafginalisation by the Khartoum government.
Analysts say the rebels are unlikely to try and take Khartoum but might force the army into a war of attrition on several fronts, draining state resources at a time when Sudan is struggling with an economic crisis. (Reporting by Ulf Laessing; Editing by Andrew Heavens)
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