Rivalry between the rebel Free Syrian Army and its nominal allies in the jihadist Al-Nusra Front, added on Tuesday to a US terror blacklist, has grown as it moves from the battlefield to the political arena.
Al-Nusra dealt a fresh blow to President Bashar al-Assad's regime on Monday, capturing the Sheikh Suleiman base, with help from allied jihadist groups.
The base was the last major military army west of Aleppo city still under army control.
But it also undercut the military influence of the more mainstream Free Syrian Army (FSA).
An AFP journalist who covered the clashes around Sheikh Suleiman said many of the fighters were from other Arab countries and Central Asia.
A rebel chief told AFP that no surface-to-air missiles were found at the base. The fighters had their hopes raised after a large cache of such arms was seized in mid-November with the capture of Base 46, another army garrison west of Aleppo.
That victory came after repeated unsuccessful attacks by a multitude of brigades. But it was an Islamist group, Al-Ansar, that precipitated the attack on Base 46, with the other rebel groups following.
The scenario was almost the same for the assault on Sheikh Suleiman -- the major difference being that this time, the jihadists launched the assault and retained control of almost the entire operation.
Only one FSA brigade, Ahrar Daret Ezza (Free Men of Daret Ezza), played a role in the offensive on the western flank of the base.
"When we saw that Al-Nusra Front began the assault, we waited for the right moment and joined the attack in our area," the group's commander Abu Jalal told AFP.
It is unclear whether there was there a "deal" made at the expense of other groups. On Monday, Ahrar Daret Ezza handed over dozens of soldiers captured during the operation to Al-Nusra Front, AFP witnessed.
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As with every key victory, a multitude of rebel groups have been quick to showcase their role in the attack through videos posted on the Internet. But Sheikh Suleiman was an undeniable victory for the jihadists.
While the insurgents said they did not secure surface-to-air missiles, as they had anticipated, Al-Nusra Front and other foreign factions did seize a large amount of arms, including anti-aircraft batteries and heavy artillery.
And the capture of the base represents the total "liberation" of a large swathe of territory west of Aleppo city and extending north along the Turkish border.
An unknown entity before the conflict broke out in March 2011, Al-Nusra first made headlines with a string of suicide bombings and only later as a disciplined fighting force, deployed on battlefields across the country.
Its successes in battle confirm its growing influence in both the anti-regime revolt and among ordinary Syrians who respect the discipline and courage of its fighters.
While the FSA has come under criticism for taking over vacant apartments and residential areas, the Islamists maintain a discreet presence out of sight.
Their success also comes at a key moment in the conflict. Under pressure from the West, rebel groups have united under a new central command, which does not include the Al-Nusra Front and fellow Islamist group Ahrar al-Sham.
The United States on Tuesday blacklisted the Al-Nusra Front, describing it as a terror group linked to Al-Qaeda in Iraq (AQI), and sanctioned its leaders.
In designating the group a foreign terrorist organisation, the State Department said that while Al-Nusra portrayed itself as part of the legitimate opposition, "it is, in fact, an attempt by AQI to hijack" the revolution.
On the ground, the FSA and other Islamist groups have had to collaborate with Al-Nusra Front, despite US efforts for it to be isolated, several rebel leaders acknowledge.
"If the West had supported us from the beginning of the revolution, this group would never have existed," said FSA commander Abu Jalal.
"At the moment, our problem is not with Al-Nusra Front, our problem is with the regime," Abu Jalal said. "Everything will be decided after the fall of Bashar al-Assad."