Turkey's president Recep Tayyip Erdogan has threatened to send "millions" of Syrian refugees to Europe in response to criticism of his military offensive into Kurdish-controlled northern Syria.

Speaking to his party, Mr Erdogan said Turkish forces had killed 109 "militants" since the operation began on Wednesday, and warned he would "open the doors" to spark a new refugee crisis in Europe if the EU called it an "invasion."

"Hey EU, wake up. I say it again: if you try to frame our operation there as an invasion, our task is simple: we will open the doors and send 3.6 million migrants to you," he said.

He added that Islamic State prisoners held by Kurdish forces would be kept in jail or returned to home countries willing to take them.

The comments came as a Syrian human rights group said up to 100,000 civilians have been displaced by fighting since Turkey launched its offensive with shelling and airstrikes against Kurdish strong points and cities on Wednesday afternoon.