Thousands of armed tribesmen were headed towards Sanaa on Thursday to back powerful opposition tribal head Sheikh Sadiq al-Ahmar, whose forces have been locked in heavy fighting with Yemen\'s security forces, tribal chiefs said. The tribesmen clashed with security forces at a military post 15 kilometres (nine miles) north of Yemen\'s capital, the sources said. The armed men \"want to enter Sanaa to back their leader\" Ahmar, one tribal chief said.   From Middle East Online