Rebels and government troops are fighting for control of a key airbase in the north of Syria, activists have said.
The Syrian Observatory for Human Rights said the rebels launched a dawn offensive to take Taftanaz airbase, a day before a crucial opposition conference in Qatar.
Taftanaz lies near the road between the capital Damascus and the northern city of Aleppo, a major front in the civil war.
Activist videos posted online claim to show the attack on Taftanaz. A rebel vehicle fires rockets and smoke rises over buildings and what appears to be an airstrip.
Activists say more than 36,000 people have been killed in Syria since the uprising began in March 2011.