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Speaking on Thursday both UN Syria envoy Staffan de Mistura and Jan Egeland, a special advisor on the Syria file, said that it was premature to rule out any scenario.

"I met with three Astana signatories," Egeland told reporters, in a reference to the Kazahk capital where last week the safe zones agreement was signed by regime-backers Russia, and Iran, in addition to Turkey, which supports opposition factions.

"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.

Reports have suggested that, under the de-escalation deal, security cordons on the edges of the established safe-zones would be established to monitor and ensure access, with forces from the three guarantor nations manning checkpoints and observation posts.

However, this has yet to be confirmed despite the plan ostensibly going into effect at midnight on Friday night.

On Thursday Egeland said that the possibility for "third parties" to carry out the monitoring remained a possibility.

For his part, also speaking on Thursday, de Mistura said that the UN had "a lot of experience" carrying out de-escalation monitoring missions, both in Syria and elsewhere, but declined to discuss specifics regarding the implementation of the deal.

The safe-zone deal calls for the establishment of four "de-escalation" zones across stretches of eight Syrian provinces.

The agreement also seeks to ground all military aviation in the four areas.

However, confusion remains over the specifics of the deal, and logistics governing its implementation.

Meanwhile, neither the Syrian government or the opposition has signed the deal.

Rebel figures have in particular expressed anger at the position of Iran - which has poured thousands of troops into the country to prop up the Assad regime - as a guarantor to the agreement.

For his part, Syrian Foreign Minister Walid Muallem has notably expressed objection to the UN playing any role in the implementation and monitoring of the agreement.

The US has given tentative backing to the deal, but remains sceptical that the Assad regime will refrain from violating its terms.

Numerous previous ceasefire deals for Syria have fallen by the wayside, amid mutual accusations from regime and rebel groups, of violations.

Speaking on Monday US Defence Secretary James Mattis said that while the US was "engaged" specific details regarding the safe-zone deal remained unclear.

"It's not been decided, in the sense that these are the specific safe zones, here's who's going to be ensuring they're safe, who is signing up for it, who is specifically to be kept out of them. 

"All these details are to be worked out and we're engaged," said Mattis, speaking in Copenhagen on Monday ahead of an anti-IS coalition meeting.

"The devil's always is in the detail."

A UN Security Council on the matter is likely to be held this week, following a request from Japan and Sweden to ascertain specific details on how the zones will work.