The Syrian National Coalition -- which is under pressure 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 three-day meeting comes as rebels face a massive onslaught by President Bashar al-Assad's forces and the Lebanese Shiite movement Hezbollah in the insurgent bastion of Qusayr in central Syria.
The opposition has long held that it can only enter into talks with members of the regime it is given international guarantees that the negotiations would lead to the fall of Assad's regime.
Some opposition members have openly expressed reservations over the US-Russian plan for a new international peace conference dubbed Geneva 2.
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"We don't have a clear picture on Geneva 2. We don't have a list of attendees, we don't know what countries are going to attend, what's the agenda, what's being proposed, what are the final goals," Coalition spokesman Khaled al-Saleh told reporters.
Saleh reiterated a long-held opposition demand that "any transitional period must start with the departure of Assad".
However, with numerous thorny issues on the Coalition's plate, Geneva 2 is not on the agenda for Thursday, opponents say.
The Istanbul meeting began after backers of the anti-Assad uprising -- including US Secretary of State John Kerry and his British counterpart William Hague -- gathered in Jordan yesterday to push for peace.
In its closing statement, the Friends of Syria group told Assad to commit to peace, warning that it would boost its backing of the opposition if he failed to negotiate a political transition.