ANKARA/PARIS (Reuters) - The leaders of Turkey, Russia, Germany and France will hold a summit on the Syrian conflict in Istanbul next week, officials said on Friday, a month after Ankara and Moscow brokered a deal to create a demilitarized zone in the northwest Idlib region.

Idlib and adjacent areas are the last stronghold of rebels who rose up against President Bashar al-Assad in 2011. The area is also home to an estimated three million people, more than half of whom have already been displaced once during the war.

Turkey and Russia reached a deal last month to set up a buffer zone running 15-20 km (9-13 miles) into rebel territory that had to be evacuated of all heavy weapons and all jihadists by Monday. The agreement averted a planned offensive on Idlib by Russia, Assad’s biggest outside backer.

The four countries will hold the talks on Oct. 27 and will discuss Idlib, and the political process for the resolution of the Syrian conflict, the spokesman for Turkish President Tayyip Erdogan was quoted by state-owned media as saying.

France wants to avoid a humanitarian catastrophe in Idlib, and thereby prevent a new exodus of refugees, President Emmanuel Macron’s office said.

“We want to keep this stability and work from this agreement to move into a new phase of discussions on a political process,” one senior French diplomatic source said. “And that it helps launch this constitutional committee that the U.N. is working on so that it meets as quickly as possible.”

German Chancellor Angela Merkel will focus on the situation in Idlib and supporting the implementation of the Sochi agreement between Russia and Turkey, a German government spokeswoman said.

“The Federal Government sees Russia, as an ally of the Assad regime, as a partner with a very particular responsibility,” the spokeswoman said.

U.N. humanitarian adviser Jan Egeland said on Thursday that Russia and Turkey plan to give more time for the implementation of the de-escalation deal in Idlib.

But he cautioned there were still “a million unanswered questions” about how the deal would work, and what would happen if groups designated as terrorists refused to lay down their weapons.

U.N. Syria envoy Staffan de Mistura said this week he would step down at the end of November for family reasons, quitting as the Syrian government - backed by Iran and Russia - has retaken most of the country and a political deal remains elusive.