KHARTOUM (Reuters) - Janjaweed militias have been concentrating forces to the north of el-Geneina, the capital of West Darfur state in western Sudan, an African Union military source said on Tuesday, corroborating a U.N. report. Janjaweed is the local name for militia forces drawn mainly from the nomadic Arab tribes of the area and blamed for much of the killing in Darfur over the past four years. "They are massing ... they have vehicles with machineguns on top and they're janjaweed. We can't say what their intentions are," said the source, who asked not to be named. The source declined...