"Since the start of our cross-border operations in Syria (in 2016), about 526,000 volunteers have returned to the safety zones that we established," Erdogan told the Turkish parliament.

Erdogan has in recent months said he is preparing to send back a million Syrian refugees on a voluntary basis.

He has said that Ankara aimed to encourage them to return to "safe zones" on the Turkey-Syria border by building them housing and local infrastructure.

There are 3.7 million Syrian refugees officially living in Turkey.

Less than nine months from presidential elections, their presence in the country has become a thorny political issue, especially as Turkey battles an economic and monetary crisis.

Opposition parties regularly call on authorities to send millions of Syrians home.

Syria's civil war, which began with a brutal crackdown of anti-government protests in 2011, has killed nearly half a million people and forced around half of the country's pre-war population from their homes.