GENEVA (Reuters) - Islamic State’s remaining Syrian strongholds are likely to fall by the end of October, which must be the trigger for the international community to push for free and fair elections, U.N. Syria envoy Staffan de Mistura said on Friday.

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“What we are seeing is in my opinion the beginning of the end of this war… what we need to make sure is that this becomes also the beginning of peace. And that is where the challenge starts at this very moment,” he said in a BBC radio interview.

Three places were still far from stabilized, de Mistura said: Raqqa, Deir al-Zor and Idlib.

“After Raqqa and Deir al-Zor, and that is a matter of a few months, there will be a moment of truth. If the international community will be helping both the opposition and the government by pushing the government to accept a real negotiation, then within a year it would be a possibility of having a truly credible election.”

The city of Deir al-Zor has been under siege by Islamic State fighters for years, forcing the U.N. to conduct an unprecedented and expensive high-altitude airdrop campaign to supply the population.

“The Syrian government and the Russians are very likely between now and the end of this month or perhaps early October, latest, to actually liberate it,” de Mistura said.

The United States and the U.S.-backed Syrian Democratic Forces “will probably liberate Raqqa by the end of October”.

The third area, Idlib, is “full of al Nusra, which is al Qaeda,” de Mistura said, referring to the Nusra Front, a one-time al Qaeda affiliate. It has been renamed and merged with other groups, but remains the only force in Syria’s war, apart from IS, that is designated by the U.N. as “terrorists”.

The lesson from the Iraqi city of Mosul, taken over by IS a decade after the United States declared “mission accomplished” in the war in Iraq, was that Syria’s war needed to be followed by a fair U.N.-managed election, without neglecting minorities.

Syria’s war has largely pitted Sunni muslims, backed by Saudi Arabia, Qatar and Turkey, against forces loyal to President Bashar al-Assad, an ethnic Alawite, who is backed by Shi’a allies, including Iran and Lebanese Hezbollah militias.

An unrepresentative peace deal would leave the door open to a resurgence of Islamic State, Sunni extremists also known by the Arabic name Daesh.

“Even those who believe they won the war – that is the government – they will need to make a gesture, otherwise Daesh will come back in a month or two months’ time.”

Nobody had an interest in a resurgence of IS in Syria, de Mistura said. Assad’s allies in Moscow, recalling the Soviet experience of war in Afghanistan, “certainly want an exit strategy.”

“We are getting close to some kind of understanding even among those who have been involved in the conflict that the priority is to close it. What we need to do is wrap it up in a way that is stabilized, not just close the conflict.”