ISTANBUL (AFP) - Syria's main opposition group opened key talks on Thursday in Istanbul to debate whether to negotiate with the regime on ending the bloody two-year civil war, as proposed by the US and Russia.

During their three-day meeting, the Syrian National Coalition - which is under fire from both its backers abroad and rebels on the ground - is also expected to choose a new president, discuss expansion to include new members and decide the fate of an interim rebel government, opposition members told AFP.

The meeting in Turkey comes as rebels face a massive onslaught by President Bashar al-Assad's forces and Lebanese Shiite movement Hezbollah in insurgent bastion Qusayr, in central Syria.

Since the Syrian conflict erupted in March 2011, more than 90,000 people have been killed in the spiralling fighting between President Assad's regime and the rebels battling to overthrow it.

The opposition has long held that it can only enter into talks with members of the regime given international guarantees that talks would lead to his fall from power.

"There is a condition that Assad resigns" under the US-Russian peace initiative dubbed Geneva 2, Coalition member Salem al-Moslet said on the conference sidelines.

However, with numerous thorny issues on the Coalition's plate, Mr Moslet added that the issue was not on the agenda for Thursday.

"But what is clear is, should the opposition's condition not be met, then I think we should call it the Lavrov conference," he said - a reference to Russian Foreign Minister Sergei Lavrov, a key Assad ally.

The meeting starts a day after backers of the anti-Assad uprising gathered in Jordan to push for peace in Syria.

The meeting included US Secretary of State John Kerry and his British counterpart William Hague.

In their closing statement early on Thursday, the Friends of Syria group told President Assad to commit to peace, warning that they would boost their backing of the opposition if he failed to negotiate a political transition.

In Istanbul, some regime opponents said it was unclear whether they would be able to make a final decision on Geneva 2 by the end of their meeting.

The Coalition is under pressure from its international backers to enter talks with the Assad regime, but if it complies, the group risks losing what little legitimacy it has left with fighters on the ground.

"The Coalition and (key opposition movement) the Syrian National Council have made clear their condition to any talks is the resignation of Bashar al-Assad," Coalition member Samir Nashar told AFP.

"I think the revolutionaries would turn their back totally on the political opposition" should this condition remain unfulfilled, he added.

But with a vast onslaught on Qusayr leaving scores dead in the past week, President Assad appears as far as ever from giving up.

In an interview with an Argentinian newspaper this month, President Assad implied that he would stay until the next scheduled election in 2014.

"The regime and its backers are trying to change the situation on the ground militarily, in order to gain the upper hand in negotiations," Nashar said.

"That's why Hezbollah and Iran are intervening in such an open way (in Qusayr). Again, this is costing the Syrians blood," he added.

At the Amman meeting, the US urged President Assad to commit to peace.

US Secretary of State Kerry said that "in the event that we can't find that way forward, in the event that the Assad regime is unwilling to negotiate... in good faith, we will also talk about our continued support and our growing support for the opposition".

Hague said that only President Assad's departure could clear the way for a settlement.

The aim of Geneva 2, the British foreign secretary stressed, would be to agree on the formation of "a transitional government with full executive authority, formed on the basis of mutual consent".

In Istanbul, dissidents also aim to name a new Coalition president to replace Mr Ahmad Moaz al-Khatib, who resigned in March, as well as three new vice-presidents and a new secretary general, a Coalition official said on condition of anonymity.

The opposition is seeking to establish a rebel government under interim prime minister Ghassan Hitto, while discussing the group's expansion to include 31 new members, the source added.

That expansion comes after pressure from Coalition backers for a more inclusive opposition, he said.

Mr Hitto has pulled together a list of ministries and representatives for all but the interior and defence portfolios - but his proposals may not even see the light as he too may end up being replaced, the official added.