Turkish President Tayyip Erdogan makes a speech at the old parliament building in Ankara, Turkey July 13, 2018. REUTERS/Umit Bektas

ISTANBUL (Reuters) - Turkish President Tayyip Erdogan told Russian leader Vladimir Putin on Saturday an accord aimed at containing the Syrian conflict could be destroyed if Syrian government forces target the Idlib region, a Turkish presidential source said.

The two presidents spoke by telephone after the Syrian government raised the national flag on Thursday over areas of Deraa in the southwest that was in rebel hands for years. The source said Erdogan voiced concern about the treatment of civilians there.

“President Erdogan stressed that the targeting of civilians in Deraa 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,” the source said.

With help from Russia and Iran, President Bashar al-Assad has now recovered most of Syria but anti-Assad rebels still control Idlib in the northwest, while a Kurdish-led militia controls the northeast and a large chunk of the east.

Turkey has set up a series of observation posts in Idlib as part of a deal which it reached last year with Russia and Iran in the Kazakh capital Astana to reduce fighting between insurgents and the Syrian government in de-escalation zones.

Erdogan said the avoidance of “negative developments” in Idlib was important in terms of encouraging rebel groups to attend a meeting in Astana planned for July 30-31, according to the source.

Separately, the Kremlin confirmed in a statement Putin’s phone conversation with Erdogan on Saturday and said they had discussed joint efforts to solve the Syrian crisis.