Recep Tayyip Erdogan says Turkey will "open the gates" for migrants to Europe if international support for a refugee safe zone in northern Syria fails to materialise.

Turkey's president said on Thursday (September 5) he plans to resettle one million refugees in northern Syria.

The country hosts more than 3.6 million registered Syrian refugees, according to the UN Refugee Agency.

It controls parts of northern Syria where it is setting up a “safe zone” with the United States. It says 350,000 Syrians have already returned and more could follow.

“Our goal is for at least one million of our Syrian brothers to return to the safe zone we will form along our 450 km border,” Erdogan said during a speech in Ankara.

"We are saying we should form such a safe zone that we, as Turkey, can build towns here in lieu of the tent cities here. Let's carry them to the safe zones there.

"Give us logistical support and we can go build housing at 30 km (20 miles) depth in northern Syria. This way, we can provide them with humanitarian living conditions.

"This either happens or otherwise we will have to open the gates," Erdogan said. "Either you will provide support, or excuse us, but we are not going to carry this weight alone. We have not been able to get help from the international community, namely the European Union."

Turkey agreed to curb the flow of migrants to Europe, under a 2016 deal between Ankara and Brussels, in return for aid amounting to billions of euros.

But renewed fighting in Idlib in recent weeks raised prospects of another wave of refugees at Turkey’s borders.

The Russian-backed Syrian army has gained a lot of ground against rebel forces, some of which are supported by Turkey after a truce failed in early August.

Nicholas Danforth, an Istanbul-based senior visiting fellow at the German Marshall Fund, told Reuters that warning about refugees in the context of the safe zone allows Erdogan to simultaneously pressure Europe and the United States.

“What seems clear is that it would be impossible to settle that many refugees in any zone achieved through negotiations with the United States and the YPG,” he said.

“This looks like an attempt to build pressure for more U.S. concessions on the safe zone, where some refugees could then be resettled for purposes of domestic (Turkish) public relations.”

Read more: Turkish President Erdogan says Syria's Idlib slowly disappearing