A failure of the opposition to unite could weaken the hand of Russia and the United States, co-sponsors of a proposed peace conference on the war, which has killed 80,000 and threatens to spill over borders and whip up wider sectarian violence.
The U.S. and Russian foreign ministers are to meet in Paris on Monday to discuss how to shepherd Syrian President Bashar al-Assad and the opposition into the talks in Geneva.
As opposition leaders met in Istanbul, Assad's forces reinforced by Iranian-backed Shi'ite Lebanese Hezbollah fighters unleashed heavy artillery and tank fire to try to seize more rebel terrain in the Sunni Muslim border town of Qusair on Saturday, sources on both sides said.
Syria is becoming a proxy conflict between Shi'ite Iran which backs Assad, whose Alawite faith is an offshoot of Shi'ism, and Arab states such as Saudi Arabia and Qatar which support Assad's mostly Sunni enemies.
George Sabra, head of the opposition Syrian National Coalition, said thousands of fighters from Iran and Hezbollah were involved in the attack on Qusair, close to the Lebanese border, and in battles in the capital Damascus.
Hezbollah leader Hassan Nasrallah said his group would stay in the Syrian conflict "to the end of the road" and would win the war for Assad's government.
"We accept this responsibility and will accept all sacrifices and expected consequences of this position," he said in a televised speech, speaking from an undisclosed location. "We will be the ones who bring it victory, God willing."
Assad's forces are believed to have seized about two-thirds of Qusair and largely surrounded the rebels. But the price was high and rebels insisted they were preventing further advances.
The insurgents see Qusair as a critical battle to preserve cross-border supply lines and deny Assad a victory they fear may give him the edge in the prospective peace talks next month.
More than 22 people in opposition-held areas were killed by Saturday afternoon, most of them rebels, and dozens wounded, according to pro-opposition monitoring group the Syrian Observatory for Human Rights. The pro-opposition Syrian Network for Human Rights said 73 people were killed by Assad's forces.
U.S. CONCERNS
The United States, concerned by the rising influence of hardline Islamists, has pressed the Syrian National Coalition to resolve its divisions and bring more liberals into the fold.
Sources at the coalition, which began its third day of meetings, said major players would focus on such international demands for a broadening of the Islamist-dominated group, leaving leadership issues for later.
Attempts to strike a grand bargain involving veteran liberal campaigner Michel Kilo and businessman Mustafa al-Sabbagh, Qatar's point man in the coalition, went nowhere in talks that stretched overnight, senior coalition sources said.
"We are back to square one," one of them told Reuters.
In Addis Ababa, on the sidelines of an African Union summit, U.S. Secretary of State John Kerry appealed to U.N. Secretary General Ban Ki-moon "to try to get something moving with respect to Syria", according to a pool reporter. Ban told Kerry he and his special Syria envoy Lakhdar Brahimi "are working very hard to convene, to make this Geneva conference a success".
Saudi Arabia, the most powerful Arab adversary of Assad, will want to see the Geneva conference, which could convene in the next few weeks, put the exit of Assad at the top of the agenda, diplomats and coalition members said.
But they said Russia, a longtime ally of Assad, wanted it to focus on a ceasefire although there is scant rapport between opposition politicians abroad and rebels inside Syria.
The inability of the coalition to alter its Islamist-dominated membership and replace a leadership damaged by power struggles is playing into the hands of Assad who, according to Russia, intends to send representatives to the peace conference.
PIVOTAL BATTLE
"The coalition risks undermining itself to the point that its backers may have to look quickly for an alternative with enough credibility on the ground to go to Geneva," a senior opposition source at the talks said.
Senior opposition figures said the coalition was likely to attend the conference, but doubted the meeting would secure their central demand - an immediate deal for Assad to quit.
While the opposition remained riven by differences, the assault by Assad's forces and their Hezbollah allies on Qusair over the past week is evolving into a pivotal battle.
Qusair controls access to Syria's Mediterranean coast, the heartland of Assad's minority Alawite community, and the battle may prove a weighty test of his ability to withstand the revolt.
Hezbollah's intervention is hardening fears that the civil war will cross borders at the volatile heart of the Middle East.
"It is ironic that Lebanon's civil strife is playing itself out in Syria. The opposition remains without coherence and the regime is intent on taking back anything it promises with violence," said one diplomat.
The diplomat was referring to a deepening sectarian divide between Shi'ite and Sunni Muslims in Lebanon, where Syrian troops were present for 29 years, including for most of the Lebanese civil war that ended in 1990.
The death toll in the northern Lebanese city of Tripoli reached 25 on Saturday in the seventh straight day of clashes between Alawite and Sunni factions backing opposing sides in Syria's war, security sources said.
