The United Nations said Thursday it was in talks with Iran, Russia and Turkey over who should control proposed safe zones in Syria, a key question after Damascus rejected any international monitors.
The UN Syria envoy Staffan de Mistura and humanitarian pointman Jan Egeland both said it was premature to rule out any scenario.
"I met with three Astana signatories," Egeland told reporters, referring to Kazakhstan's capital where the safe zones pact was signed by Russia and Iran, which back the Syrian regime, and opposition supporter Turkey.
"What they say is that we now sit down and agree, they will agree with our input on whom should be controlling security (and) the monitoring," he said.
Forces from the three countries were one option for monitoring, as were "third parties", he added.
De Mistura, speaking at the same press conference, said the UN had "a lot of experience" when it comes to such monitoring but declined to discuss specifics on implementing the deal.
Syrian Foreign Minister Walid Muallem has rejected any UN role in monitoring the designated areas.
The May 4 agreement calls for the creation of four "de-escalation zones" to shore up a ceasefire, ban flights and allow for deliveries of humanitarian aid.
"We do have a million questions and concerns, but I think that we don't have the luxury that some have of this distant cynicism of saying it will fail," Egeland said.
"We need it to succeed", he added.
The agreement has not been signed by the Syrian government or the opposition.
Japan and Sweden have requested a UN Security Council meeting to obtain specific details on how the zones will work.
Uruguay's UN Ambassador Elbio Rosselli, who currently holds the council presidency, told reporters in New York Thursday that the meeting will take place "probably next week".
De Mistura will host a sixth round of Syria peace talks set to open on Tuesday in Geneva.
He said he expected the negotiations to last just four days, far shorter than previous rounds which have typically stretched to 10 days.
While his office has cautioned that major breakthroughs remain unlikely, de Mistura said the safe zones deal had created some momentum and that he "wanted to hit the iron while it's hot ... after the Astana meeting".
The UN hoped to inject "a touch of the will of optimism" into the upcoming round, he added, even if previous rounds have yielded almost nothing concrete.