Syrian rebels are reported to have been battling government troops inside a key military airport near Aleppo for the first time.
The UK-based Syrian Observatory for Human Rights said they launched a dawn raid and heavy fighting was taking place. Rebels claim to have killed soldiers and captured tanks.
The Free Syria Army hopes that gaining control of airbases would enable it to disrupt government supplies. Meanwhile it has rejected calls from Sunni clerics in Lebanon for a jihad in the central Homs region.
More fighting has also been reported in Damascus.
New video uploaded onto the internet purported to show destruction in the suburbs of Daraya and Jobar after shelling by forces loyal to President al-Assad.
Earlier this week activists said hundreds of civilians were feared dead after government shelling on the mainly Sunni Muslim suburb of Jdeidet al-Fadel.
The Syrian authorities accuse rebels of inflicting casualties and say they are fighting “terrorists”. State media said seven people were killed and more than two dozen injured in a mortar-bomb attack on a government-held suburb of Damascus.
The government has called on the international community to take its side.
Meanwhile the Pope has called for the release of two Orthodox bishops kidnapped in Syria, and for blood to “stop being spilt”. Greek Orthodox archbishop Paul Yazigi and Syriac Orthodox archbishop Yohanna Ibrahim are both based in Aleppo.
Claims that they had been freed on Tuesday have been denied. It is not known who abducted them.