The Syrian National Coalition is also expected to elect a new president during the closed-door proceedings that opened in Istanbul.
The meeting comes amid a push by Russia, a key supporter of President Bashar Assad, to try to bring the Syrian government and the opposition together in Moscow for talks to end a civil war that has killed more than 200,000 people.
Russian diplomats have been shuttling between the sides in recent weeks to sound out their willingness to attend the meeting that the Kremlin has said it hopes to convene after Jan 20.
Assad's government has said it's ready to attend "preliminary" talks, but says they should pave the way for a conference in Syria itself.
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Russia has said that the first stage of talks would include members of both the government-tolerated internal opposition and opposition groups based abroad, including the Coalition. In the next stage, they would be joined by Syrian government representatives.
So far, there is little to suggest that the UN plan or the Russian initiative has a real chance of success. But analysts say there is a greater chance now for a settlement as a result of recent government losses in the north and south as well as growing resentment toward Assad among his traditional supporters particularly after the mass killings of soldiers by militants.