Russia and China have joined western powers in calling on Damascus to allow the UN’s humanitarian chief immediate access to the country as Syrian rebels retreated from the besieged Bab Amr district of Homs after more than three weeks of bombardment.

With one of the bloodiest episodes in the year-long uprising against Bashar al-Assad’s regime appearing to reach a turning point, a rare unanimous statement by the UN Security Council – which has been paralysed by Russian and Chinese resistance to pressure on the regime – expressed disappointment at Damascus’s refusal to allow a visit by Lady Amos.

The Free Syrian Army, the loosely affiliated group of defected soldiers and other armed opponents of the regime, said in a declaration circulated on the internet that it had opted for a “tactical retreat” to spare thousands of civilians. It cited “worsening humanitarian conditions, lack of food and medicine and water, electricity and communication cuts as well as shortages in weapons”.