Turkey could face an influx of 2-3 million more Syrian refugees if President Bashar al-Assad's forces or Islamic State insurgents advance around Aleppo, Turkish Foreign Minister Mevlut Cavusoglu said on Tuesday.
U.S. warplanes have been bombing Islamic State forces in parts of Syria, but Assad's military has intensified its campaign against some rebel groups in the west and north that Washington sees as allies, including in and around Aleppo.
"Right now who is filling the void left by IS as a result of the coalition's air attacks? It is the regime," Cavusoglu told a news conference in Ankara.
"But there is not that much difference between the IS and the regime. Both kill cruelly, especially civilians. And neither hesitate to use whatever weapons are available to them."
Rebels and Syrian government forces hold parts of Aleppo, Syria's most populous city before the war. Islamic State has seized territory from rival Islamist groups in a belt of territory north of Aleppo, threatening rebel supply routes.
Islamic State also holds large sections of territory elsewhere in the wider Aleppo province.
Turkey has been a staunch supporter of the Free Syrian Army, an umbrella term for dozens of armed groups fighting against both Assad and Islamic State. It has been pushing for the U.S.-led coalition to broaden its campaign to also tackle Assad.
"A weakening of the moderate opposition, the FSA backed by the coalition, ... will make the adverse situation in Syria worse and more unstable," Cavusoglu said.
"Fearful civilians are fleeing from areas where IS, terrorist groups and the regime are gaining ground. A possible advance in Aleppo would mean the influx of two to three million people to the Turkish border to seek asylum."
The Syrian civil war has killed close to 200,000 people and forced more than 3 million refugees to flee the country, according to the United Nations.
Turkey already hosts more than 1.5 million refugees and has been pushing the United States and its allies to create a safe haven for displaced civilians on the Syrian side of its border.