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 said more than 36,000 people have been killed in Syria since the uprising began in March 2011.
Meawnhile, Israel's military said three Syrian tanks entered the demilitarised zone in the Golan Heights.
A military spokeswoman said Israel complained to the UN peacekeeping force in the area after the tanks entered on Saturday.
The relatively low-key response suggested Israel did not see the move as an immediate threat.
The Israeli news site Ynet said the tanks and two armored personnel carriers drove a few miles away from Israeli military positions.
There are concerns in Israel that violence from Syria's civil war could spill over a long-quiet frontier. Misfired Syrian shells have exploded inside Israel on several occasions. Israel captured the Golan Heights from Syria in the 1967 Mideast war and annexed it.