Credit: BBC
Rebels and regime forces have been battling for control of several air bases in Syria.
The government said it had repelled a huge attack on an air base near Aleppo while rebels claimed victory in another battle in the east of the country.
The rebels have increasingly targeted the air force in recent weeks, accusing it of launching attacks on cities with helicopter gunships and fighter jets.
Meanwhile, Algerian Lakhdar Brahimi has taken up his post of UN peace envoy.
Mr Brahimi, who has taken over the role from Kofi Annan, has sought to play down expectations for his mission.
UN Secretary General Ban Ki-moon has been lobbying international leaders for support, saying he “believes in the power of diplomacy”.