Failure to agree on who will take part in an international conference on the Syria could thwart hopes that the make-or-break meeting will be held in June, diplomats said Wednesday.
UN leader Ban Ki-moon said that key "elements" of the proposed conference had still not been agreed by the United States and Russia.
US ambassador to the United Nations, Susan Rice, also acknowledged that which countries will take part as well as the representation of the Syrian government and opposition had still to be hammered out.
But Rice told reporters that the US government's "entire foreign policy apparatus" was working with Russia to hold the meeting.
Whether Iran and Saudi Arabia take part and who will represent the divided Syrian opposition and President Bashar al-Assad's government have all clouded preparations for the meeting.
The main opposition coalition has insisted at meetings in Istanbul this week that any talks with the government must lead to Assad's resignation. Divisions within the coalition have become clear at the talks however.
US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov agreed this month to press for the conference, aiming to find a political solution to the 26-month-old conflict that activists say has left more than 94,000 dead.
The talks would follow up on a meeting of the major powers held in Geneva in June last year that set a transition process. This time, it would include representatives of President Bashar al-Assad and the Syrian opposition.
But talks between Kerry and Lavrov in Paris on Monday failed to break deadlock on the details of the conference, according to western diplomats.
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Lavrov said after the Paris meeting that it would be a "tall order" to get the conference organized. He has also said US support for a UN Human Rights Council resolution on Syria, passed on Wednesday, had undermined peace efforts.
Ban said he spoke with Kerry on Wednesday and that "active consultations" on the conference were still going on between the United States, Russia and the United Nations.
But he said there are "still many elements which we have to clear," and stressed that the largely divided opposition must go to any conference "in a coherent and unified manner."
"We also have some other issues, participation issues -- who should participate in this meeting," the UN chief added, without giving details.
Russia has called for Iran, a key backer of Assad, to be allowed at the meeting. But western nations say Iran, which was not at the last Geneva meeting, should be excluded.
Ban said he could not give a date for the conference but added "we are all committed to convene this meeting as soon as possible."
In a separate message to a meeting in Tehran on the Syria crisis, Ban said the proposed Geneva conference was the "best opportunity" for a negotiated settlement. "The challenges ahead are formidable but we cannot afford to miss this opportunity," he said.
Western nations had hoped to hold the conference in Geneva during the week of June 10. "It's not clear whether that will happen or not, there's obviously a risk of slippage," said one western diplomat.
A meeting between senior Russian and US diplomats this week is expected to prove crucial in deciding whether the conference will be in June, the diplomat added.
The US ambassador to the United Nations said there were "outstanding questions and issues" on which countries will take part, as well as the government and opposition. Rice said there was no agreement yet on the timing of the meeting.
Kerry "has been working very hard on this, as has the entire foreign policy apparatus of the United States government," said Rice.