A Free Syrian Army fighter runs over a wooden plank with his weapon in Aleppo’s Karm al-Jabal district Nov. 3, 2013. (Saad Abobrahim/Reuters)

Forces loyal to Syria’s government are taking advantage of deepening rifts among the country’s rebels to advance into rebel-held territory in northern Syria, overturning some long-held assumptions about the war.

The resignation Sunday of a top leader in the U.S.-backed Free Syrian Army further underscored the extent to which rebel infighting is undermining the effort not only to topple President Bashar al-Assad but also to hold on to territories won by the opposition in more than two years of conflict.

Col. Abdul Jabbar Akaidi, one of the chief recipients of the limited U.S. aid provided to the opposition, said he was standing down to protest the rebel bickering, which he blamed for the capture Friday by Assad loyalists of the strategic town of Safira, southeast of the key city of Aleppo.

The fall of Safira restored a vital supply link between Damascus and government forces holding out in the divided northern city and put regime loyalists on track to challenge other opposition strongholds in Aleppo province, almost all of which has been under rebel control for more than a year. Opposition commanders said Safira fell after Islamist brigades failed to respond to a call for reinforcements by the Taw­heed Brigade, Aleppo’s biggest rebel battalion, which was forced to flee under a withering bombardment by the Syrian air force.

Peace talks at stake

The government’s advances in the north call into question some of the received wisdom about the state of play on the Syrian battlefield, a constantly shifting procession of skirmishes, sieges, offensives and counteroffensives that for many months has amounted to a stalemate. While the government has reinforced its hold over the central provinces of Homs and Damascus in recent months, the rebels have been consolidating their grip in the north, a divide that is expected to underpin peace talks that the United States and Russia are hoping to sponsor in Geneva this month.

The battle of Safira is unlikely to signal a decisive shift in the pattern. The government’s supply lines are long and vulnerable to rebel ambush. But it does suggest that the rebels are more vulnerable than had been thought in the vast swaths of northern territory that they captured last year, analysts say. If the rebels are unable even to hold ground in the north, the calculus of the Geneva peace conference also will shift, said an opposition figure who spoke on the condition of anonymity because the issue is sensitive. The opposition will be less inclined to attend if it is losing, he said, and “Assad will have no reason to compromise at all.”

Panic spread across rebel-held Aleppo over the weekend as residents who thought the government had been expelled for good contemplated the prospect that their neighborhoods could be contested again.

“People are so scared,” said Zakaria Ahmed, 39, an Aleppo businessman who traveled Sunday to the Turkish border town of Kilis after a night of intense shelling by government forces triggered fears of an imminent offensive. “It seems the Free Syrian Army is incapable of defending even themselves, let alone the people.”

‘Lack of unity’

The capture of Safira fits a pattern in recent months in which the government has reversed rebel gains with the help of an influx of thousands of Shiite fighters from Iran, Iraq and the Lebanese Hezbollah movement, reinforced by steady supplies of weapons from Russia and Iran. The rebels complain that they receive barely a fraction of such support from their Western and Persian Gulf backers, known as the “Friends of Syria.”

But rebel defeats in the north can be attributed as much to disarray within the ranks of the opposition as to the fighting capacities of the government, analysts say.

“It’s a consequence of their lack of unity of command and purpose. The regime is united and purposeful,” said Jeff White, defense analyst at the Washington Institute for Near East Policy.

Clashes have been erupting with increasing frequency between mainstream units of the Free Syrian Army and fighters with the al-Qaeda-affiliated Islamic State of Iraq and Syria, which has expanded rapidly across the rebel-held north in recent months. Units loyal to Gen. Salim Idriss, the U.S.-backed leader of the Free Syrian Army’s Supreme Military Council, “have been fighting a two-front war, which has seriously hampered their efforts against the regime,” the U.S. ambassador to Syria, Robert Ford, testified during a Senate Foreign Relations Committee hearing last week.

At the same time, powerful Islamist battalions that do not adhere to al-Qaeda’s ideology but fall outside the structures of the Supreme Military Council have been forming new alliances dedicated to the creation of an Islamic state, rather than a democracy. They have received the lion’s share of funding from wealthy Persian Gulf countries and are positioning themselves to receive more in the wake of pledges by Saudi Arabia to counter U.S. inaction in Syria by increasing aid to the rebels.

At a gathering of top commanders at a remote hotel outside the southern Turkish city of Reyhanli last week, the moderate Free Syrian Army representatives rebuffed an attempt by the Islamists to join the military council, accusing them of seeking to displace the mainstream units, which have inferior resources. But commanders at the meeting also acknowledged that they are increasingly outgunned and outmanned by the Islamist brigades.

Akaidi, among those whose influence has waned as the Islamists have expanded, criticized the stream of meetings being held to thrash out the differences. “You warlords . . . stop chasing positions and stature and only caring about being famous,” he said in a video posted on YouTube. “Return to the battlefield and leave behind your egos.”