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"The current regime is responsible for the way things have evolved in Syria... I don't think it's a realistic prospect to build lasting peace in Syria with Assad (in place)," he said.
The European Union is "entirely confused" since Britain voted to leave the bloc and it needs to revisit its vision for enlargement and Turkey's place in that, Yildirim also said.
Launched in 2005 after decades of seeking the formal start of an EU membership bid, Ankara's membership negotiations have long been sensitive for France and Germany because of Turkey's status as a large, mainly Muslim country.
The large purge that President Tayyip Erdogan has carried out following a failed coup attempt in July 2016 has worsened relations between Brussels and Ankara.
During a visit to London, Yildirim said the European Union needed to look again at its plans to expand the bloc.
"After the Brexit decision the EU is entirely confused. They need to revisit their vision for the future, how far they are going to enlarge and what place Turkey will have in that. We are here. We are not going anywhere," he said.