Already weakened by political infighting, Syria's opposition has been dealt another blow by the posting online of videos purporting to show rebel fighters committing atrocities, analysts say.
And on the back foot due to army advances on the ground, the opposition is also under international pressure to enter into dialogue with President Bashar al-Assad's regime.
Videos posted online that showed a rebel mutilating a soldier's corpse, and of a jihadist summarily executing 11 Assad supporters "will undermine the opposition's narrative of an uprising against a dictator", said Swedish expert on Syria, Aron Lund.
"Even though they are not representative of the whole of the insurgency, such footage puts the opposition in a difficult position as it draws attention to the opposition's abuses... and undermines their chances of getting support from Western nations", Lund told AFP.
The videos also shed new light on the sectarian nature of the Syrian conflict, with victims often apparently targeted because of their religious affiliation.
While most rebels -- like the population -- are Sunni Muslims, Assad and many of his military commanders adhere to the minority Alawite sect, an offshoot of Shiite Islam.
Aside from the videos, abuses committed against members of particular faiths by followers of other religious beliefs have frequently been documented by rights groups.
Most recently, the Syrian Observatory for Human rights reported the killing of 145 people in a Sunni district of the coastal city of Banias on May 3 by regime troops and militiamen.
The watchdog said the slaughter came a day after some 51 others were killed in the nearby Sunni town of Bayda.
The videos showing rebel atrocities have been met with international condemnation, further raising concerns over arming the rebels, for fear such weapons may fall into the hands of extremists.
"The opposition is under two kinds of pressure. It is being told to accept a political solution with members of the regime, and to limit... the presence of Islamists among its ranks," said Ziad Majed, who teaches at the American University of Paris.
"This makes both the situation on the ground and relations with some of the opposition's backers more difficult," Majed told AFP.
"Meanwhile, the regime steps up the savagery of its operations on the ground... while many divisions mar and weaken the opposition."
Opponents to Assad "seem to be in a perpetual state of organising themselves", Salman Shaikh of the Brookings Institution told AFP.
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The main opposition National Coalition now has to decide whether to take part in an international conference called by Moscow and Washington to push for a political solution, and also to choose a new chief, said Shaikh.
"The opposition is put into a difficult position in that it is going into this conference (set for June) without any real assurances -- if it goes," he added.
Though they support opposite sides in the Syria conflict, the United States and Russia have made a joint call for a political solution.
Their call was adopted by the other UN Security Council members as well as by Turkey and the Arab League.
The proposed talks are an extension of the Geneva agreement last year, which did not specify that Assad had to step down as part of any solution to the crisis.
The main opposition National Coalition, which insists that Assad's departure is a key condition for a political solution, will decide in an Istanbul meeting on May 23 whether it will take part in the international conference.
Assad has made it clear he does not intend to leave power before his mandate ends in 2014.
On the ground, the army has escalated its operations and made advances near Damascus and in the central province of Homs, strategically located near the Alawite heartland.
Hundreds of fighters from Lebanese Shiite movement Hezbollah, who support Assad, have helped the army in Qusayr near Lebanon and in Sayyida Zeinab near Damascus.
Although large swathes of northern Syria are under rebel control, insurgents lack the weapons they need to overrun army bases that they have besieged for months.
Moscow's military support to Damascus has meanwhile remained strong. It opposed a UN General Assembly resolution Wednesday that condemned the regime's escalation, and has continued to provide new heavy weapons.
"Undoubtedly, the key backers have reinforced their support and doubled and tripled it," said Sheikh.
Damascus' main regional ally Tehran "is the cause of many of the army's military advances", said Riad Kahwaji, a Dubai-based military expert.
"The advances on the ground are small-scale, not strategic", Kahwaji told AFP.
"One cannot say the regime has made a strategic shift and that it is winning on the ground. We are still very far away from that."