Rebels in northern Syria on Wednesday seized most of a military base, their third a major battlefield success in as many days, as Russia said it will host talks with both the regime and opposition.
As prospects faded for a political solution to the war that has killed nearly 70,000, UN chief Ban Ki-moon called on the Security Council to overcome its paralysis and take "meaningful" action to stop the bloodshed.
Secretary of State John Kerry said, meanwhile, that the United States was aiming to change Assad's belief that he can hang onto power and accept "the inevitability" of his departure.
At least another 145 people were killed across Syria on Wednesday, including 66 civilians, said the Syrian Observatory for Human Rights, which also reported the rebels' seizure of most of the military base in Aleppo province.
The majority of Base 80 "has come under insurgent control" a day after the rebel fighters launched a coordinated assault on two airports that the strategic facility's troops are tasked with securing.
Dozens of rebels and forces loyal to President Bashar al-Assad were killed, said the Britain-based group which relies on a network sources on the ground for its information.
The insurgents on Tuesday overran a military air base at Al-Jarrah, also in Aleppo province, after taking control of Syria's largest dam in the neighbouring province of Raqa the day before.
Activists have said Aleppo's insurgents shifted their focus from targets in the provincial capital to military bases because they are a source of ammunition and weaponry, and to put out of action warplanes used to bomb rebel bastions.
In Aleppo city itself, meanwhile, electricity and water supplies were down for a fourth day, said the anti-regime Aleppo Media Centre, warning of a "humanitarian disaster" in what was once Syria's commercial hub.
Commenting on the rebels' advance in Aleppo province, Observatory director Rami Abdel Rahman said the army may be giving up on parts of northern Syria in order to secure the centre.
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"The army is barely resisting the rebels' advance on bases in the north. Meanwhile, it is resisting with ferocity the insurgents in Daraya (southwest of Damascus) and Homs (central Syria)," Abdel Rahman told AFP.
"The regime understands it cannot survive a transition without securing some land to bargain with," he added.
Despite the advances, regime warplanes carried out several air raids on rebel areas in Aleppo province while army tanks shelled the east Damascus district of Jobar, the Observatory said.
Insurgents have secured enclaves in the eastern and southern suburbs of Damascus, and the army is trying hard to push them out.
Dozens of people in neighbouring Lebanon cut off two northern border crossings with Syria, meanwhile, in protest at the transfer of diesel fuel tanks to the Syrian regime, demonstrators said.
The crowd gathered at the Tartus and Aabudiyeh crossings leading to the Syrian province of Homs and blocked trucker routes by piling stones and other objects, forcing dozens of Syrian cars, buses and trucks to back up.
"We will not allow the passage of gasoline and diesel fuel tankers, because this fuels goes to the military efforts of the regime of Bashar al-Assad," Moin al-Meraabi, a member of Lebanon's anti-Damascus Future Movement, told AFP.
The conflict broke out when President Assad's forces launched brutal crackdown on democracy protests that erupted in March 2011, inspired by the Arab Spring uprisings.
A top Russian diplomat said Syrian Foreign Minister Walid al-Muallem and Ahmed Moaz al-Khatib, the head of the opposition umbrella National Coalition, would make separate visits to Moscow for talks in the coming weeks.
Speaking after talks with Jordan's foreign minister, Secretary of State Kerry hinted the US and Jordan could take renewed steps to urge Russia, a key Syria ally, to bring more pressure on Assad to quit.
"I still remain hopeful that there may be an equation where the Russians and the United States could in fact find more common ground than we have found yet with respect to that," Kerry said, adding that Jordan's King Abdullah II was expected to visit Moscow.