GENEVA (Reuters) - Syria’s opposition must accept that they have not won the six-and-a-half year war against President Bashar al-Assad, U.N. peace talks mediator Staffan de Mistura said on Wednesday.

De Mistura suggested the war was almost over because many countries had got involved principally to defeat Islamic State in Syria, and a national ceasefire should follow soon after. The main rebel-held area, the city of Idlib would be “frozen”.

“For the opposition, the message is very clear: if they were planning to win the war, facts are proving that is not the case. So now it’s time to win the peace,” he told reporters.

Asked if he was implying that Assad had won, he said pro-government forces had advanced militarily, but nobody could actually claim to have won the war.

“Victory can only be if there is a sustainable political long-term solution. Otherwise instead of war, God forbid, we may see plenty of low intensity guerrilla (conflicts) going on for the next 10 years, and you will see no reconstruction, which is a very sad outcome of winning a war.”

De Mistura plans to join ceasefire talks in the Kazakh capital Astana next week, which he said should help resolve the fate of Idlib, a city of 2 million where rebels designated as terrorists by the United Nations are gaining influence.

“I am confident...there will be a non-conflictual solution - let’s say not a new Aleppo, that’s what we want to avoid at any cost, if we have learned from the past,” he said, referring to the greatest battle and humanitarian crisis of the war.

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“If that takes place Idlib may become frozen in a way in order to avoid becoming a major tragic end to the conflict.”

Meanwhile Islamic State was facing imminent defeat in its two main zones of influence - in the city of Raqqa and around the city of Deir al-Zor.

“The fact is that Deir al-Zor is almost liberated, in fact it is as far as we are concerned liberated, it’s a matter now of a few hours.”

Raqqa’s fall would follow within days or weeks, leading to the “moment of truth” for a round of negotiations in October.

Since early 2016, de Mistura has overseen a tortuous series of peace talks in Geneva that has made almost no visible progress. He said the next round could see an accelerated and more pragmatic approach.

“The issue is: is the government, after the liberation of Deir al-Zor and Raqqa, ready and prepared to genuinely negotiate and not simply announce victory, which we all know, and they know too, cannot be announced because it won’t be sustainable without a political process?

“Will the opposition be able to be unified and realistic enough to realize they did not win the war?”