Syrian troops bombarded the outskirts of Damascus, where monitors said almost 50 people died in a battle for a military airport, as the country's 20-month conflict homes in on the capital.
Russia and France, meanwhile, prepared for talks on Tuesday in Paris at which Russian Prime Minister Dmitry Medvedev and France's President Francois Hollande are expected to address differences over the Syria conflict.
"There is a major disagreement," Russia's envoy to Paris, Alexander Orlov said. "The West says (a solution) must start with the departure of (President) Bashar al-Assad, and we say this is where it must end."
Turkey and close Syria ally Iran, whose positions on the crisis are also diametrically opposed, held closed-door talks on Saturday.
No details were published after a two-hour meeting in Istanbul between Iran's parliament speaker Ali Larijani and Turkish Prime Minister Recep Tayyip Erdogan, whose country Tehran accuses of arming the rebels.
World powers have made no progress on finding a political solution to the conflict, with Russia and China blocking UN Security Council efforts to ratchet up the pressure on Damascus.
In the face of the revolt, Assad's regime has been reducing its territorial ambitions to focus on Damascus, central Syria and Alawite bastions, as it digs in for a long war, according to analysts.
On the ground, rebels captured a "large part" of the military airport of Marj al-Sultan, 15 kilometres (nine miles) east of Damascus, and destroyed two helicopters overnight, the Syrian Observatory for Human Rights said on Sunday.
The Britain-based monitoring group, which relies on a network of activists, lawyers and medics for its information, said 31 rebels and 16 soldiers were killed in the battle.
Meanwhile, regime helicopter gunships pounded the northeast and southwest outskirts of Damascus as clashes also spilled over into southern districts of the capital, the Observatory said, giving a preliminary toll of 28 people killed in Damascus province on Sunday.
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Although the conflict is often seen as one between an opposition led by the country's Sunni Arab majority and the minority Alawite-dominated regime, the fault lines in Damascus are as a much a matter of social class as of confession or ethnicity.
Rebels have set up rear bases in orchards surrounding the capital, where they had made advances during the summer but have since been driven out.
In northern Syria, rebels pressed on with an offensive against troops stationed at the strategic Tishrin dam, which straddles the Euphrates River and connects the provinces of Aleppo and Raqa.
Rebels already control one of the main routes to Raqa, and the dam would give them a second passage, connecting a wide expanse of territory between the two provinces, both of which border Turkey.
Insurgents also surrounded the military airport of Deir Ezzor city in the east, where four rebels and a civilian were killed in clashes, sniper fire and shelling.
The army has already lost much of the eastern part of Deir Ezzor province, which borders Iraq. Just over a week ago, the rebels seized control of a military airbase in the border town of Albu Kamal.
In the southern province of Daraa, insurgents took control of a military outpost on the border with Jordan overnight, but later vacated the area for fear of air strikes that regularly follow rebel gains, the Observatory said.
Meanwhile, it said, five people died in a bus explosion in the province.
The watchdog said 55 people were killed killed nationwide on Sunday, including 27 civilians.
In other developments, the Observatory said rebels seized the training camp of a pro-regime Palestinian faction, the Popular Front for the Liberation of Palestine-General Command, near Damascus.