The Syrian rebel force fighting government troops in the besieged city of Homs says most of its fighters have left the Baba Amr district in a "tactical" withdrawal. The Free Syrian Army (FSA) said it was pulling out of Baba Amr to spare the lives of the 4,000 civilians who were refusing to leave their homes. Baba Amr has been under heavy bombardment for nearly a month. Syrian government forces say they now have full control of the district. The head of the FSA, Col Riyad al-Assad, said government troops had already moved in and were combing the area. Appeal to Red Cross The BBC's Jim Muir, in neighbouring Lebanon, says the withdrawal was obviously by agreement between the two sides, as has happened elsewhere in Syria, to avoid a final showdown. A few fighters remained behind in Baba Amr to cover the retreat, the FSA said. A statement posted online in the name of the Baba Amr brigade of the FSA said the fighters did not have enough weapons to defend the civilians. It calls on the Red Cross to enter Baba Amr immediately to help all civilians still there. The number of civilians remaining in Baba Amr, which normally has a population of 100,000, cannot be independently confirmed. The statement also warns the government forces not to harm civilians, saying the opposition would take severe revenge if they did. It has been snowing heavily in Homs, slowing the advance of ground troops which began their offensive on Tuesday, but also worsening conditions for civilians. Many of those still in Baba Amr are without power and running low on basic supplies. Hundreds of people died during the month-long bombardment, activists say. Opposition split Meanwhile, the exile political opposition Syrian National Council (SNC) says it has formed a military bureau to co-ordinate the various armed anti-government groups. However, our correspondent says the FSA is refusing to accept the authority of the new bureau - evidence of deep splits within the opposition. Announcing the creation of the new bureau, SNC leader Burhan Ghalioun said the uprising had begun as a non-violent movement, but the council had to "shoulder its responsibilities in light of this new reality". Mr Ghalioun said the bureau would function like a defence ministry and would be staffed by soldiers from the main armed group, the Free Syrian Army, as well as civilians. Meanwhile, the UN's rights council has passed a resolution condemning "systematic violations" against civilians and calling for aid agencies to be allowed access. China and Russia, which both vetoed two UN Security Council resolutions in Syria, voted against the proposal, as did Cuba. The vote carries no legal weight, but analysts say it may embolden diplomats to take a tougher line in Security Council debates.