Recep Tayyip Erdogan said the zone - which is already being set up in co-operation with the US - needed to be extended for the goal to be met.
US-backed Kurdish fighters earlier moved back from a strip of Syrian territory along the Turkish border.
Turkey regards the Kurdish forces as terrorists.
Mr Erdogan's comments came after talks in Ankara with the presidents of Russia and Iran, Vladimir Putin and Hassan Rouhani.
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