United Nations Secretary General Ban Ki-moon acknowledged progress has been slow in arranging international peace talks aimed at ending Syria's civil war.
Mr. Ban said Wednesday key elements of the U.N.-sponsored talks are still undecided, including whether the Syrian opposition will attend and who would represent it.
"We have yet to find out, agree, mutually agreeable and convenient date for a meeting, and also we have yet to see how the opposition groups are discussing their unity issues. We expect that opposition group will come in a coherent and unified manner as a single representations."
Earlier, Syrian opposition leaders repeated a demand for President Bashar al-Assad to leave power, saying they will not agree to the talks without a timetable for his withdrawal.
The Assad government has agreed, in principle, to attend. But on Wednesday, Syrian Foreign Minister Walid al-Moallem insisted Mr. Assad would remain president until at least the 2014 elections, and that he may run for another term.
The United States and Russia agreed earlier this month to organize the talks, which would bring rebel and government forces together for the first time since the conflict began 26 months ago.
The Syrian National Coalition, the country's main opposition bloc, meets in Istanbul on Thursday for a final day of talks that have been characterized by sharp disagreements about its leadership and direction.
A day earlier, the coalition laid out a series of preconditions for entering the international peace conference. A coalition statement said the group wants "binding international guarantees" that Mr. Assad will not be a part of any transitional government.
Analysts and diplomats say it may not be possible to hold the Geneva conference in early June, as tentatively planned, if both sides continue show little willingness to give ground.
The U.N. says the 26-month civil war, which began as a largely peaceful protest against Mr. Assad's government, has killed at least 80,000 people.