Syria's opposition factions, struggling under Western and Arab pressure to close their ranks and elect a viable leadership, have left a meeting aimed at creating a coherent front key to a proposed international peace conference.

"They have delayed the crucial vote on the expansion of the Syrian coalition ," said Al Jazeera's Hashem Ahelberra, reporting from Istanbul on Saturday.

After three days of meetings in Istanbul, senior coalition players were in discussions late into the night on Friday after Michel Kilo, a veteran liberal opposition figure, rejected a deal by Mustafa al-Sabbagh, a Syrian businessman who is the coalition's secretary-general, to admit some members of Kilo's bloc to the coalition, the sources said.

Kilo has said that his group wants significant representation in the opposition coalition before it will join.

"Kilo is ready to join but his list includes 25 people in a take or leave offer," said our correspondent, Hashem Ahelberra.

"The problem with the opposition is that if they add the group of secularists into the general committee, they will have a veto power, and right now the current opposition thinks the secularists have been very soft on Assad, and they might undermine the hardliners and the Islamists."

Much to the frustration of its backers, the coalition has struggled to agree on a leader since the resignation in March of Moaz al-Khatib, a former Damascus religious leader, who had floated two initiatives for Assad to leave power peacefully.

Khatib's latest proposal, a 16-point plan that sees Assad handing power to his deputy or prime minister and then going abroad with 500 members of his entourage, won little support in Istanbul, highlighting the obstacles to wider negotiations.

"He has the right to submit papers to the meeting like any other member, but his paper is heading directly to the dustbin of history. It is a repeat of his previous initiative, which went nowhere," a senior coalition official said.

The failure of the Syrian National Coalition to alter its Islamist-dominated membership as demanded by its international backers and replace a leadership undermined by power struggles, appears to be playing into the hands of President Bashar al-Assad.

No let up in violence

By Saturday night, the factions locked themselves up in a room, trying to find a way to work together.

And while they continued their discussions behind closed doors, fighting continues inside Syria in Qusayr, where heavy bombardment has been going on for days.

Interim opposition leader George Sabra spoke at a press conference in Istanbul on Saturday, when he took a harsh tone with Hezbollah as well as Iran.

"Thousands of invaders from the Iranian forces and the terrorists of Hezbollah are still coming to Syria and still killing our people," said Sabra.

"The killers are blockading, shelling and trying to storm several cities...they are, with the participation of the falling Syrian regime, killing Syrians in so many locations, in all governorates," said Sabra.

Meanwhile, in a speech on Saturday, Hezbollah chief Hassan Nasrallah on Saturday vowed "victory" in Syria, where fighters of his powerful Lebanese Shia movement are fighting alongside regular troops against rebels trying to topple the regime.

"I say to all the honourable people, to the mujahedeen, to the heroes: I have always promised you a victory and now I pledge to you a new one" in Syria, he said at a ceremony marking the 13th anniversary of Israel's military withdrawal from Lebanon.

Government forces are attacking a key town as Assad's ally Russia says he will send representatives to a proposed international conference in the Swiss city of Geneva, coalition insiders said.