Turkey has expressed fears another two to three million Syrian refugees could cross its borders if the region around the city of Aleppo is overrun either by Islamist extremists or Syrian government forces.
Turkey is already hosting at least 1.5m refugees displaced by the Syrian conflict and has repeatedly warned that its capacities are being strained by the numbers.
Foreign minister Mevlut Cavusoglu said supporting the rebel Free Syrian Army was the only option for the international community against what Turkey sees as the twin threat of Islamic State miltants and the regime of President Bashar al-Assad.
"The main force fighting both ISIS and the Syrian regime today is the Free Syrian Army," he said, using another term for the IS group.
However, Mr Cavusoglu told reporters the FSA "has failed to achieve the desired outcome because it is fighting against both groups."
Mr Cavusoglu said there was little difference between IS militants and Mr Assad's regime.
"Both of them are killing people brutally and don't refrain from using any kinds of weapons at their disposal. Both force people to flee their land."
He added: "An advance on Aleppo would mean an influx of two to three million people to the Turkish border."
He said a weakening of the moderate opposition to Mr Assad and the FSA would "result in the advance of the unstoppable ISIS as well as the regime".
"And this will make Syria even more unstable. Therefore, the advance of both of them should be halted."
Turkey has repeatedly called for the ousting of Mr Assad as the sole way to resolve the Syrian crisis permanently.
However, it has grown increasingly concerned in recent months that the US-led coalition strikes against IS could end up strengthening Mr Assad's regime.
Turkey has been seeking to persuade the US that a three-pronged approach is needed to strike against IS, Syrian government forces and Kurdish militants.
However, it is unclear if its arguments have made any headway with the Us government.
In recent months, Mr Assad's forces have advanced around the outskirts of the eastern portion of Aleppo that is under rebel control, threatening to encircle it completely.
Rebel-held areas of Aleppo are under the control of multiple groups, including fighters affiliated with the FSA.
