This claim originates with the Russian defence ministry and is set out, for example, here. The claim was made to explain the renewed shelling of the remaining rebel enclave today, despite yesterday’s ceasefire and evacuation plan.This bus story must have been exclusively a press release as the claim is repeated by numerous media organs but cannot be found on a Russian government website. However, Russia’s Centre for Reconciliation of Opposing Sides in the Syrian Arab Republic has just made available its 14th December bulletin – these bulletins come out at 8pmGMT most evenings. The business of the buses is not referred to in today’s disquisition here. The claim stated that rebels attacked the buses in the Salah ad-Deen (صلاح الدين) zone. That area is referred to in the issue, but without reference to evacuation buses, or any buses. A quick look at the current map, as provided by a pro-government source, will demonstrate the improbability of rebels seeking to break out towards the north-west:
However, the map shows what the compiler understands to be an intended destination for evacuated rebels: Anadan (عندان). If true, then the rebels attacked buses (Salah al-Din is shown on the map) in order to break out in the direction in which they were due to be evacuated anyway, possibly in the same buses – a four-kilometre hike along a north-bound dual carriageway through government territory the most likely route for the break-out. These rebels, however, exist to defend the trapped citizenry or, with the citizenry, evacuate. They would achieve nothing by making a suicidal run for it, which is what the press release suggested, by firm implication, their attack on the buses presaged. Thus, this press release did not achieve its intended effect. What it recounts is too improbable.
The reason for the resumption of shelling of the remaining rebel enclave may be seen clearly in this distasteful video clip, kindly retweeted by the guy who makes the maps (@):
However, that Russia needed to invent a reason is interesting. It means they had not been briefed. There may be a little bit of hope concealed in that omission.