[Northern Monkey]

'Last month in a Congressional hearing, General Joseph Dunford, chairman of the US Joint Chiefs of Staff, was asked what he thought about the imposition of a no-fly zone to halt the barrel-bombing in Aleppo.
His response was clear. "Right now", said the highest ranking US serving officer, "for us to control all of the air space in Syria would require us to go to war against Syria and Russia; that's a pretty fundamental decision…"
Russia effectively controls Syrian air space, having deployed highly capable S-400 air defence missiles and radars at its Syrian base.
It has now brought in S-300 missiles, which have an additional capability against other missiles.
And Russian warships in the Mediterranean could add to the air defence shield. Enforcing a no-fly zone would require a willingness to down Russian and Syrian aircraft in a highly contested environment.
Typically any US operation to institute such a zone would require enemy air defences to be destroyed at the outset. A US strike against Russian SAMs (surface to air missiles) is surely unthinkable.
Russian spokesmen have warned the Americans off - making it clear that any attack even on Syrian government forces by the Americans will be viewed as an attack on Russia.'

From the BBC