Syria's main opposition group is meeting in Istanbul to consider possible negotiations with President Bashar al-Assad's regime to end the two-year conflict.
The United States and Russia have proposed organizing peace talks involving the opposition and Assad's government.
The joint U.S.-Russian initiative, which is dubbed Geneva 2, aims to build on last year's international agreement in Geneva that outlined plans for a transition of power in Syria but broke down over disagreements about Assad's role.
Opposition members told the media that during the three-day meeting in Turkey, the Syrian National Coalition is also expected to choose a new president and discuss expansion to include new members.
However, some participants said it was unclear whether they would be able to make a final decision on Geneva 2 by the end of their meeting.
The gathering comes a day after the group of backers of the Syrian opposition known as Friends of Syria gathered in Jordan to push for peace.
The meeting included U.S. Secretary of State John Kerry and his British counterpart, William Hague.
Kerry, speaking May 23 in Israel at a meeting with Prime Minister Benjamin Netanyahu, warned about the serious regional impact of the Syrian civil war.
"Nobody has any illusions about how difficult, complicated, what a steep climb that is," Kerry said. "But we also understand that the killing that is taking place, the massacres that are taking place, the incredible destabilization of Syria, is spilling over into Lebanon, into Jordan, and has an impact obviously on Israel. So, we have an obligation to try to see if we can implement [last year's Geneva agreement]."
Kerry said a major concern is the flow of advanced weapons from Assad's allies into Syria.
"S-300 missiles coming from Russia or other countries, Iran missiles, are destabilizing to the region," Kerry said. "The United States is committed, not only in its defense of Israel, but in its concerns for the region to try to address this issue."
UN estimates say more than 80,000 people have been killed since the Syrian conflict erupted in March 2011.
With reporting by AFP and Reuters