Battles continued between Syrian regime forces and rebels for the control of a key highway outside Damascus, activists have said, as rebels launched fierce assaults in several other parts of the country.
Sunday’s fighting was the heaviest in the capital since the first rebel push into the city last July.
Checkpoints on the main artery into the city have changed hands several times since Wednesday, when opposition fighters started their campaign for the capital, the seat of President Bashar al-Assad's power.
A rebel fighter told the Associated Press that opposition forces on Sunday overran the al-Adnan checkpoint in Jobar, northeast of Damascus.
Regime forces are battling to retake control of an 8km- belt around the capital. Opposition fighters are in control of most towns around Damascus, but have so far been kept away from the centre.
Elsewhere, Syrian rebels launched a fierce attack near Deir Ezzor where they used tanks to shell Brigade 113 just north of the eastern city, according to reports.
Brigade 113 and the besieged military airport are some of the last regime holdouts in the city. The outlying province is largely held by the rebel fighters.
The opposition Deir Ezzor press network, a grassroots group of activists, said that the humanitarian conditions in the province were dire.
"The western countryside is out of water and electricity supplies for the eleventh day in a row due to the indiscriminate shelling by the regime forces," it said.
Two weeks ago, the rebels captured the vital Siyasiyeh suspension bridge that straddles the Euphrates river in Deir Ezzor, cutting off the main regime supply route to the adjoining province of Hasakeh.
Regime troops have already been forced from the vast territory stretching from Deir Ezzor city to the Iraqi border, including the border town of al- Bukamal and its small military airbase.
In the northeast province of Raqqa on Sunday, fighters from Jabhat al-Nusra, a self-proclaimed Jihadist group, along with other rebel battalions, took over an army company in Tabqa along the Euphrates river, securing a major cache of artillery and ammunition and taking control of a key checkpoint in the town.
Rebels also continued their assault on Wadi Deif military base, a major regime holdout in the largely rebel-held northwest province of Idlib, while the army retaliated shelling the nearby town of Maaret al-Numan.
Syria's conflict began in March 2011 with largely peaceful protests inspired by the Arab Spring revolts elsewhere in the region. It evolved into a civil war as the opposition took up arms to fight a government crackdown on dissent.
The fighting has settled into a bloody stalemate and shows no signs of stopping, despite several tentative proposals from both sides to find a peaceful resolution to the conflict.
The violence on the ground came as Syrian opposition leader Mouaz al-Khatib met with international peace envoy Lakhdar Brahimi in Cairo on Sunday. He is set to meet Nabil Elaraby, the secretary-general of Arab League, on Monday.
Late last month, Khatib, the head of the Syrian National Coalition, changed course and said he is willing to talk to the regime if that would help end bloodshed. He suggested that Assad release tens of thousands of political prisoners as a first step.
In-depth coverage of escalating violence across Syria
Syria's Information Minister Omran al-Zoubi floated the latest proposal late on Friday, saying Damascus is ready for dialogue with the opposition so long as the rebels lay down their weapons. He said anyone who responds will not be harmed.
The offer is unlikely to gain any traction among the Syrian opposition. The rebel movement is highly decentralised and deeply distrusts the regime, and most groups are unlikely to stop fighting so long as Assad remains president.
The opposition in January rejected a proposal that Assad put forward for ending the conflict, although it would have kept him in power. He offered a national reconciliation conference, elections and a new constitution. He also dismissed any chance of dialogue with the armed opposition and called on Syrians to fight what he called "murderous criminals".