Thousands of Syrians, mostly women and children, have fled toward the Turkish border since Friday from the northern Syrian city of Aleppo to escape a major regime offensive backed by Russian air strikes.
"The regime has now blocked a part of Aleppo... Turkey is under threat," Erdogan told reporters on his plane returning from Senegal on Saturday.
"If they reached our door and have no other choice, if necessary, we have to and will let our brothers in," he said.
The governor of Turkey's Kilis border province, Suleyman Tapsiz, said Saturday that Turkey -- already home to 2-2.5 million Syrians -- was taking care of 30-35,000 refugees who had gathered around the nearby Syrian city of Azaz in the space of 48 hours.
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Another 70,000 may head for the frontier if Russian air strikes and Syrian regime military advances continued in Aleppo, he added.
Turkey's Oncupinar border crossing, which faces Bab al-Salama inside Syria, remained closed to thousands of refugees gathered there for a third day, an AFP reporter said.
"But the border keeps open for emergency situations," a Turkish offical told AFP.
"Seven injured were taken to Turkey on Friday and one on Saturday for treatment at Turkish hospitals," he said.
Turkish Foreign Minister Mevlut Cavusoglu said Saturday his country would keep its "open border policy" for Syrian refugees.