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A Syrian regime assault on the rebel-held Idlib regions risks destroying peace efforts at Astana, Turkish President Recep Tayyip Erdogan reportedly told Russian leader Vladimir Putin on Saturday.

The pair spoke by telephone two days after the Syrian regime raised the national flag over areas of Daraa in the southwest that was in rebel hands for years and the cradle of the 2011 popular uprisings.

"President Erdogan stressed that the targeting of civilians in Daraa was worrying and said that if the Damascus regime targeted Idlib in the same way the essence of the Astana accord could be completely destroyed," a Turkish presidential source told Reuters.

Despite sitting on opposite sides, Russia and Turkey, alongside Syrian President Bashar al-Assad's ally Iran, have been working to reach a political solution through the peace talks based in the Kazakh capital Astana.

The Syrian regime with the help of its allies has recaptured most of the country but anti-Assad rebels, largely the jihadist Hayat Tahrir al-Sham group, still control Idlib in the northwest, while a Kurdish-led militia controls the northeast and a large area of the east.

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Turkey runs a series of observation posts in Idlib as part of a deal reached last year with Russia and Iran in Astana to reduce fighting between insurgents and the Syrian regime in de-escalation zones.

Erdogan said it was important to avoid "negative developments" in Idlib in order to encourage rebel groups to attend a meeting in Astana planned for 30-31 July, according to the source.

Speaking to °®Âþµº earlier this month, Professor Joshua Landis, the director of the Center for Middle East Studies at the University of Oklahoma, said it was logical Assad will target Idlib next.

"Once the Daraa operation begins to wind down, Assad will move his elite troops up toward Idlib," he said. "But the north will be a very different battle." 

To bring the whole of the country under his heel, Assad "will want to push the tens of thousands of rebel soldiers who have collected in Idlib province into Turkey, along with their families".

"Turkey will not accept this," added Landis. "Already Turkey has set up  in Idlib that are manned with Turkish soldiers and heavy equipment."

Ultimately, Landis anticipates Russia playing "a crucial role" when it comes to "negotiating an agreement" between the Syrian and Turkish militaries in Idlib, while "Assad will probe and push to see how much land in Idlib province he can retake".