In recent weeks, rebels have held a large swath of territory in northern Syria along the border with Turkey, a vital supply route, and seized several military bases, including key airfields.
At the same time, the rebels have held onto several districts of Damascus despite heavy counterattacks by some of Assad's best troops to the extent that the regime has had to mount airstrikes against parts of Damascus, the seat of its power.
Oxford Analytica says the "stalemate between government and rebel forces is set to break in 2013."
It observed in an analysis published Monday: "The balance of power has begun to tip in the rebels' favor with their attacks growing, the political opposition forming a new and more effective body and the government losing control of increasing swathes of territory.
"However, the regime remains united and is escalating its use of heavy weapons and air power."
Even so, analysts and regional diplomats say, it's becoming clearer that after 20 months of bloodletting in which up to 40,000 people have been killed, the grip of the regime, dominated by the minority Alawite Muslim sect, is weakening across the Sunni-majority country.
"The regime has failed to dislodge opposition militants from Aleppo," Syria's commercial capital and long a regime stronghold, "and the Damascus suburbs," Oxford Analytica noted.
The Syrian army was forced to abandon its last base in Aleppo, Syria's strategically important second city and the central battleground of the war, in early November.
That's only one of several major rebel gains in recent weeks that U.S. global security consultancy Stratfor says illustrates "shifting dynamic in the fighting and a clear erosion of the regime's strategic position in northern Syria."
Indeed, it could lead to the rebels cutting off Damascus from the Alawite-dominated Mediterranean coast, the regime's bolt hole.
The regime's elite and most loyal units, led by the Republican Guard and the 4th armored division commanded by Assad's younger brother Maher, clearly can no longer control large parts of the country.
"Rebel groups cannot defeat the regime but are able to attack military bases, economic targets and intercept main communications routes," Oxford Analytica commented.
However, it added, "a reorganized opposition will enable greater Western support -- however, this will fall short of military intervention.
"The civil war will intensify and the regime of President Assad is likely to collapse by the end of 2013."
Western powers, led by the war-weary United States, don't want to be dragged into an increasingly savage conflict that could trigger violent unrest in neighboring Lebanon, Turkey, Jordan and Iraq.
This could spread even further across a region that's been in turmoil since pro-democracy uprisings broke out in early 2011 and brought down four Arab dictatorships.
But the Persian Gulf monarchies, which feel threatened by the ill-named Arab Spring and by Iran, Syria's key ally, are taking a harder line and providing arms and funding for the rebel groups.
Saudi Arabia and Qatar, in particular, are determined to bring down Assad's regime to end Shiite Iran's drive to extend its power across the Sunni-dominated Arab world.
Islamist-led Turkey has allowed rebel forces to base in its territory and has provided arms and funding as well, one reason why the opposition is able to hold ground in the north.
Deep-seated rivalries within the Syrian opposition have prevented the cohesion of rebel forces since the uprising against Assad erupted March 15, 2011.
This has also allowed Salafist jihadists, such as al-Qaida, to gain a foothold in Syria.
Their fighters, many of them veterans of other wars, are the most effective of the rebel forces. They have proven to be formidable foes of the regime because they're heavily financed by power brokers in the gulf.
But the opposition, increasingly desperate for Western support, overt or otherwise, finally set aside its rivalries in Qatar Nov. 11 to boost the prospect of getting rid of Assad, whose family has ruled Syria since 1970.
The formation of the National Council for Revolutionary and Opposition Forces under a moderate leadership is likely to boost rebel links with the West and possibly pave the way for a government-in-exile.
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