By Khaled Yacoub Oweis
ISTANBUL (Reuters) - Syria's opposition cast doubt on Friday on hopes of any meaningful progress in planned international peace talks after President Bashar al-Assad said only a referendum could decide whether he should leave power.
Russia and the United States are trying to draw Assad's representatives and his opponents into a conference in Geneva on forming a transitional government in an effort to end a civil war that has killed more than 80,000 people.
The acting head of the opposition coalition said it would stay out of any such talks as long as Lebanese Hezbollah guerrillas were fighting alongside Assad's forces.
It was not clear if the statement by George Sabra was the fragmented organization's final word.
"The Syrian coalition will not participate in international conferences and will not support any efforts in light of Hezbollah and Iran's militia invasion of Syria," Sabra said.
Russia, Assad's main ally, has accused the Syrian opposition for its part of undermining the prospects for negotiations by voting to take part only if a deadline is set for an internationally guaranteed settlement based on Assad's exit.
The Syrian National Coalition has been riven by disagreements during a week of talks in Istanbul, and Sabra's colleagues were more cautious.
A spokesman said the coalition had not made a final decision on whether to go to Geneva. Other officials said Sabra's view did not necessarily represent that of the wider opposition.
"Not going means handing victory to the regime, which will appear as a peace dove," said Kamal al-Labwani, a long-time political prisoner turned outspoken advocate for armed struggle.
"We have to perform well publicly in the talks, which has to turn into a public trial for the regime and its crimes ... We have to make the removal of the regime fundamental to any political solution," he told Reuters.
In an interview with Lebanon's Al-Manar television broadcast on Thursday, Assad said anything agreed at the Geneva talks would have to be approved by the Syrian people, including him handing over any powers.
"Either side can propose anything but nothing can be implemented without the approval of the Syrian people," he said.
"Changing the authorities of the president is subject to changing the constitution; the president cannot just relinquish his authorities, he doesn't have the constitutional right. Changing the constitution requires a popular referendum."
The opposition, under pressure to broaden its Islamist-dominated leadership, struggled to overcome deep rifts during the talks this week in Istanbul and form a united front for the proposed Geneva conference.
Sabra said an offensive by Assad's forces, supported by Iranian-backed Hezbollah fighters, to capture the border town of Qusair had "dampened" hopes of reaching a political solution.
"It is difficult to continue when Syrians are constantly being hammered by the Assad regime with the help of outside forces like Iran and Russia," he said.
Delegates in Istanbul agreed to add 14 named members of a liberal bloc led by veteran figure Michel Kilo to the 60-member assembly of the Syrian National Coalition, the closest body that Assad's foes have to an overall civilian leadership.
The coalition also agreed in principle to admit another 14 members of activist groups from inside Syria and 15 members linked to the Free Syrian Army, an umbrella organization for anti-Assad fighters.
The Free Syrian Army command will meet in Istanbul on Saturday to choose its representatives to the coalition.
The changes could strengthen their hand at Geneva, but a neutral mechanism to choose these 29 new members has not been agreed and another battle to name them is shaping up for the next coalition meeting on June 12, members said.
"The whole idea behind the expansion is to make the opposition stronger for Geneva," said a diplomat who monitors Syrian opposition politics, but added more time was needed.
"I think whatever happens, Geneva will be postponed at least until July. No one is really ready."
(Additional reporting by Erika Solomon in Beirut; Writing by Nick Tattersall; Editing by Mark Heinrich)