Air strikes by regime and Russian aircraft on rebel positions in the northwestern province of Idlib kill at least 21 civilians, including eight children, a monitor says .
At least 28 people are reported to have been killed in an air strike on a refugee camp in rebel-held northern Syria, monitors say.
Turkey again summons the Russian ambassador after a second violation of its airspace by a Russian warplane operating in Syria in two days.
US President Barack Obama says the Russian bombing campaign in Syria in support of President Bashar al-Assad is driving moderate opposition underground and "only strengthening" Islamic State.
