Damascus (AFP) - Syrian Foreign Minister Walid Muallem said on Monday his country would reject any United Nations role in monitoring the implementation of four "de-escalation" zones.

"We do not accept a role for the United Nations or international forces to monitor the agreement," Muallem told reporters in Damascus.

Regime backers Russia and Iran and rebel supporter Turkey reached a deal on Thursday on four "de-escalation zones" in Syria where the government and opposition will halt hostilities.

The deal says those areas would be bordered by "security zones" with checkpoints and observation posts "ensured by the forces of the guarantors by consensus", but that "third-party" monitors could also be deployed.

Muallem on Monday said there could be a role "as the Russian guarantor has said, for military police", but it was unclear if he was referring to Syrian or foreign units.

The multi-phase plan, signed Thursday in the Kazakh capital Astana, is one of the more ambitious efforts aimed at ending Syria's six-year conflict.

It provides for a ceasefire, rapid deliveries of humanitarian aid and the return of refugees after "de-escalation zones" are created across stretches of eight Syrian provinces.

The Syrian government and rebel groups are not signatories to the deal, although Muallem on Monday reiterated Damascus's approval of it.

But, he stressed, "if any violations take place, the Syrian army will be prepared to respond in a decisive manner".

The agreement covers four main battlegrounds between the government and non-jihadist rebels -- the northwestern province of Idlib, parts of Homs province in the centre, the south, and the opposition enclave of Eastern Ghouta near Damascus.

It does not include territory where clashes are raging against the Islamic State group in Syria's centre, north and east.

IS is coming under simultaneous attack by Syrian government troops as well as a US-backed alliance of Kurdish and Arab fighters known as the Syrian Democratic Forces (SDF).

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Muallem said on Monday that the SDF's fight against IS was "legitimate", in the Syrian government's first sign of approval of those operations.

"In this phase, we believe that what Kurdish citizens in Syria are doing by fighting Daesh is legitimate, in the framework of protecting Syria's territorial unity and sovereignty," he said, using the Arabic acronym for IS.

More than 320,000 people have been killed since Syria's conflict erupted in March 2011.