The Syrian opposition said on Wednesday it would only take part in planned international peace talks if a deadline was set for a settlement that forces President Bashar al-Assad to leave power.
In its first official reaction to the Geneva conference being prepared by the United States and Russia, the opposition coalition adopted a declaration calling for "binding international guarantees" for any resolution of Syria's two-year-old conflict.
The statement, issued after seven days of meetings riven by internal dispute, demanded "the removal of the head of the regime and the security and military command".
The talks have been marred by disagreement within the coalition over expanding its membership and appointing a new leadership. Lack of unity has threatened to rob the Islamist-dominated alliance of international support.
The 60-member coalition has so far failed to agree on the wider involvement of a liberal opposition bloc, to the dismay of Western and some Arab backers keen to reduce Islamist influence.
Further evidence of dissent among the rebels emerged on Wednesday when opposition groups in Syria accused their counterparts in exile of undermining the rebellion and lacking legitimacy.
Dismayed by the "ongoing discord", a statement by four leading opposition groups in Syria dismissed attempts to expand the coalition as having "no real impact on the revolution" and said at least half the coalition's leadership bodies should be made of "revolutionary forces".
The statement, issued in the name of the Revolutionary Movement in Syria, said it could not "bestow legitimacy upon any political body that subverts the revolution".
The coalition's failure to agree even the basic structure of its membership bodes ill for a unified stance on the peace talks, which aim to agree a transitional government and an end to a conflict that has killed 80,000 people.
"There is a daunting realisation that the opposition has to get its act together before Geneva, otherwise the Assad team will run rings around us," a senior opposition coalition source at the talks in Istanbul said.
Diplomats say the Geneva talks could be held in mid-June, but officials in the Middle East say they will be pushed back to July.
Syria's foreign minister, Walid al-Moualem, said his country had no preconditions for attending although it had yet to decide who would represent it.
While the opposition coalition has bickered in Istanbul, Assad's forces have been pressing a fierce counter-offensive on the ground in Syria.
Backed by Hezbollah -- said by French intelligence to have provided up to 4,000 fighters -- Assad's army is fighting to dislodge rebels from a stronghold on the Lebanese border.
His troops have already pushed back rebels in the southern province of Deraa and retaken some outlying areas east of Damascus, consolidating their hold from the capital up to the coastal heartlands of his minority Alawite sect.
Syrian state television said government forces had seized an air base near the town of Qusair on Wednesday.
Russia, which has shielded Assad diplomatically since the Syrian uprising erupted in March 2011, says it will deliver an advanced S-300 air defence system despite U.S. and French objections, saying it would deter "hotheads" intent on foreign intervention.
In Geneva, the United Nations Human Rights Council voted to call on Syria to halt its unlawful attacks on civilians in rebel-held Qusair, but the resolution highlighted deep divisions among powers ahead of the planned peace talks.
The U.N. human rights chief, Navi Pillay, earlier urged countries not to supply Syria with weapons and to seek a political end to the war.
Assad's opponents have suffered from deep-seated rifts since the start of the uprising. The opposition in exile has little influence over activists on the ground, while the only authority that Syrian army defectors in Turkey and Jordan have over the hundreds of rebel brigades scattered across Syria stems from their ability to channel weapons from abroad.
Ideological differences between Islamists and nationalists are exacerbated by conflicting ambitions of backers as diverse as the Gulf Arab monarchies, the United States and Europe.
At the heart of the stalemate in Istanbul is a rivalry between regional backers of the rebels centred around Qatar, which supports the Islamist coalition members, and Saudi Arabia, which is wary of the Islamist Muslim Brotherhood.
The addition of a liberal bloc of more than 20 seats, led by Christian opposition campaigner Michel Kilo, had been intended to ease the Islamist grip over the coalition. Kilo has so far been offered just five seats.
International envoys sought to break the impasse in Istanbul, with Saudi Prince Salman bin Sultan meeting Kilo to discuss his demands for representation.
If a deal is not struck, coalition insiders say the liberal wing will not participate in peace talks, further threatening the ability of the coalition to speak for the opposition.
The coalition had meant to discuss a new leadership in Istanbul, including the fate of provisional Prime Minister Ghassan Hitto, who has not been able to form a provisional government in exile since being appointed on March 19.
George Sabra, the acting head of the coalition, appeared intent on dropping the membership issue and proceeding to electing a new leadership. But other senior opposition officials said such a move would only deepen divisions.
"If they go ahead with choosing a new leadership, they are setting the stage for a war between Qatar and Saudi Arabia, and nobody wants this," one of the officials said.
(Additional reporting by Gulsen Solaker in Ankara; Writing by Dominic Evans; Editing by Will Waterman and Giles Elgood)