In a speech to a business forum on Thursday, Erdogan also said that Turkey had information that Iran-backed forces in Syria were carrying out "merciless massacres".
He said Turkey will be patient up to a point over the crisis in Syria and will then be forced to take action.
Tens of thousands of Syrians have fled to the border with Turkey following a Syrian government offensive around the rebel-held city of Aleppo backed by Russian airstrikes.
"There is a chance the new wave of refugees will reach 600,000 if air strikes continue. We are making preparations for it," Erdogan said.
"We will show patience up to a point and then we'll do what's necessary. Our buses and planes are not waiting there in vain," he said.
Turkey, already home to more than 2.6 million Syrian refugees, has long pushed for the creation of a safe zone in northern Syria to protect displaced civilians without bringing them over the border into Turkey.
The proposal has so far gained little traction with Washington or NATO allies who fear it would require an internationally patrolled no-fly zone which could put them in direct confrontation with Assad and his allies.
Erdogan said the Syrian crisis could not be resolved without safe zones, and said that ways of keeping Syrians in their country needed to be sought.
Erdogan also said he had previously told the EU's two top officials, Jean-Claude Juncker and Donald Tusk, that the time could come when Turkey would open the gates for migrants to travel to Europe.
"In the past we have stopped people at the gates to Europe, in Edirne we stopped their buses. This happens once or twice, and then we'll open the gates and wish them a safe journey, that's what I said," Erdogan said in a speech.
A Greek news website said on Monday that Erdogan, in a meeting with Juncker and Tusk, had threatened in November to flood Europe with migrants if EU leaders did not offer a better deal to help Turkey manage the refugee crisis.
Want to enjoy 'Zen' reading - with no ads and just the article? Subscribe todaySubscribe now