Syria's civil war, with its large-scale defections, thousands of soldiers killed and multiple fronts, has eroded one of the Arab world's biggest armies, with pro-Assad militias increasingly filling in for troops.
But while the rebels have scored military and diplomatic gains, the regime is far from its breaking point.
Assad appears to have stopped trying to retake all of the rebel-held areas, lacking the manpower to do so. But his forces have pinned down opposition fighters with artillery and airstrikes, while repelling rebel assaults on the capital of Damascus and other regime strongholds.
In this scenario, the regime can hang on for months, said Joseph Holliday, a Syria analyst at the Washington-based Institute for the Study of War. "The opposition is definitely ascendant, and Bashar is going down, (but) it's a question of time," he said.
Syria's troop strength moved into the spotlight with a call for a general mobilization by Grand Mufti Ahmad Badreddine Hassoun, the country's top state-appointed Sunni Muslim cleric and Assad loyalist. He told state TV on Sunday that Syrians must rally to defend their country against a "global conspiracy."
On Tuesday, the pro-government al-Watan newspaper dismissed speculation that the mufti's appeal was a sign of attrition among the troops. The army is "fine" and troops "have been waging since for the past two years with unprecedented valor and courage," the newspaper said in a commentary. The army can keep fighting for years, it asserted.
Experts say precise figures on rebel and regime troop strengths are difficult to come by. The Syrian military does not release detailed information and last year stopped publishing data on soldiers killed.
Rebel groups often operate locally, with considerable autonomy, despite attempts by Syria's main opposition group to introduce a centralized military command.
The uprising against Assad began two years ago, initially peacefully. In response to a regime crackdown, it turned into an armed insurgency and finally, last summer, into a full-scale civil war. The conflict has killed more than 70,000 people and displaced about 4 million of Syria's 22 million people, according to U.N. estimates.
The Syrian army had about 220,000 troops at the start of the conflict, according to Holliday, who follows battlefield developments in Syria.
Assad only deployed the most loyal one-third of those soldiers, or between 65,000 and 75,000, to try to beat back the insurgency, Holliday estimated. Tens of thousands more deserted, while others were confined to their barracks as unreliable, he said in a new report.
Assad and Syria's ruling elite are members of the Alawite minority, an offshoot of Shiite Islam, while most of the rebels and a majority of army conscripts belong to the country's Sunni majority.
Estimates vary on casualties among the troops.
As of Tuesday evening, the Britain-based Syrian Observatory for Human Rights said it had documented the deaths of 14,521 soldiers. The activist group issues daily detailed updates on casualties on both sides.
Syria analyst Jeffrey White estimated that an average of 40 Syrian soldiers are killed every day, or about 1,200 a month. He said this is based on the analysis of funeral data, but he declined to elaborate.
Holliday and White said the standard calculation is that for each soldier killed, four are wounded, which means the military loses hundreds, if not thousands, additional fighters to injury each month.
"The regular army has suffered significant attrition over time," said White of The Washington Institute for Near East Policy think tank.
With troops stretched thin, the regime has largely attacked rebel positions from the air or with artillery, while concentrating its ground troops around key strongholds.
This allowed rebels to take control of territory in the north and the sparsely populated east of the country, where they captured the provincial capital of Raqqa, their first city, earlier this month.
Over the weekend, rebels seized a missile base in the Damascus suburb of Khan Sheih, west of Damascus, killing at least 30 soldiers, opposition groups said Tuesday. Rebels said they seized nine anti-aircraft guns and other ammunition.
And on Tuesday, 40 soldiers were killed, including eight slain when a car bomb exploded on the outskirts of the city of Idlib. The blast wounded 10 people, including a correspondent for the pro-regime station Ikhbariyeh, according to the Observatory.
The regime's units in northern Syria appear to be heavily depleted by combat losses and desertion.
For example, rebels believe, based on deserters, that there are only about 350 soldiers at the Wadi Deif military base near the embattled town of Maaret al-Numan, even though the base has dozens of armored vehicles, which would normally require many more soldiers to run and maintain.
Rebels also drive freely through most of the northwestern Idlib province, where the regime has abandoned or lost many village-level garrisons or highway outposts and withdrawn inside large bases.
The army is being reinforced by pro-regime militias. This includes the "shabiha," or Alawite shock troops, and "popular committees" that have sprung up in Shiite and Christian areas supportive of the regime.
White said the militias are believed to have tens of thousands of fighters. "We see them more involved in combat, in an offensive role, not just sitting at checkpoints," he said. Some of the armed men are receiving military training, he said.
Even women have begun joining the militias—an unusual step in this conservative region.
A government official in the central Homs district said Tuesday that about 100 women have joined militias in Homs and the country's largest city, Aleppo, and are operating checkpoints. He spoke on condition of anonymity because he was not permitted to speak to the media.
A U.N.-appointed committee investigating war crimes in Syria said members of the popular committees have been involved in house-to-house searches, identity checks, mass arrests and looting. Mass killings with sectarian overtones have also been attributed to such groups, the U.N. panel said Monday.
Fortifying the militias "is a move by the government to supplement its own manpower, as it begins to lose some of the manpower that it used to have," said panel member Karen Koning AbuZayd.
Holliday said that while Assad is unlikely to regain control over Syria, he is in a position to keep fighting. Even if he is eventually toppled, his loyalists could mount a fierce insurgency against the new rulers, keeping the country at war for years to come, he said.
Associated Press writers Steve Negus in Cairo, John Heilprin in Geneva and Albert Aji in Damascus contributed to this report.