Turkish President Recep Tayyip Erdoğan has warned Russian counterpart Vladimir Putin against any pro-Assad assault on opposition-held Idlib Province in northwest Syria.

Erdoğan’s office said on Saturday that the President spoke of the destruction of a Turkish-Russian agreement for “de-escalation” in the northwest.

An official also said that Erdoğan expressed concerns about the treatment of civilians in Daraa Province in southern Syria, overrun by pro-Assad forces in a three-week offensive. Russia enabled the advances with airstrikes, breaking a de-escalation zone that it proclaimed with the US last July.

“President Erdoğan 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,” his office said.

Through the Astana process, named after the Kazakh capital where meetings were held, Russia, Turkey, and Iran had declared agreements for cooperation.

However, the Assad regime — propped up by Russia and Iran — has sworn that it will retake “every inch” of the country, and Tehran has publicly backed the intention to regain Idlib, almost all of which has been held by the opposition since spring 2015.

Turkey, alongside rebel forces, regained parts of northern Syria from August 2016. First the Turkish-rebel offensive pushed out the Islamic State, then from January 2018 the attacks took much of the Kurdish Afrin canton.

Turkey has set up a series of 11 observation posts around Idlib and parts of western Aleppo and northern Hama Provinces.

The Kremlin made no reference to the Idlib warning in its summary of Saturday’s phone call, merely saying Putin and Erdoğan discussed joint efforts to solve the Syrian crisis.