DAMASCUS — The Syrian military said on Thursday a unilateral ceasefire backed by Russia had come into force to allow people to leave besieged eastern Aleppo, a move rejected by rebels who say they are preparing a counter-offensive to break the blockade.

State media earlier said the army had opened exit corridors in two designated areas in the Bustan al Qasr quarter and near the Castello road in northern Aleppo city. Waiting buses were shown on state television.

Intensified Russian and Syrian bombing of besieged rebel-held parts of Aleppo has killed hundreds.

The United Nations has criticized unilateral ceasefires after long sieges, saying they can be helpful only if combined with humanitarian access for those who do not want to leave.

The 250,000 civilians trapped inside the besieged rebel held parts of the city have so far stayed away from the corridors. The army blames rebels opposed to President Bashar al-Assad for preventing them leaving and says they use civilians as human shields. The government says the militants are holding Aleppo residents as hostages. 

Rebels say the goal of Moscow and Assad is to empty rebel-held areas of civilians so they can take over the whole city.

“They talk about humanitarian corridors, but why are they not allowing food into besieged eastern Aleppo to alleviate our suffering? We only need the Russian bombers to stop killing our children. We don’t want to leave,” said Ammar al Qaran, a resident in Sakhour district.

State-owned Ikhbariyah television said rebels had fired a mortar barrage near to where ambulances had been heading to take patients from the besieged parts of the city for treatment in government-held areas.

Russia President Vladimir Putin said on Thursday that Moscow was ready to extend the humanitarian pause in Aleppo “as far as possible”.

“We have made clear our intention to extend as far as possible, depending on the current situation on the ground, the halt in our air strikes,” Putin said during a press conference broadcast on Russian television following negotiations on Syria with French leader Francois Hollande and German Chancellor Angela Merkel in Berlin.