Turkey will never let another Qandil [terrorist headquarters belonging to the PKK] form on its southern border, President Recep Tayyip Erdoğan said on Wednesday, while he slammed Russia, the U.S. and the Assad regime for openly supporting the PYD, recognized as a terrorist organization by Ankara.Speaking to district governors at the Presidential Palace, President Erdoğan said that Turkey's cross-border operations may be limited under the scope of engagement rules, but these rules may be stretched in the future if there is a threat against Turkey's national security."We will take necessary measures against structures which pose a security threat to our country," Erdoğan said.He criticized Russia for committing war crimes, and slammed the United Nations for waiting to look for 'evidence'."What else are you looking for? The hospitals are being bombed," Erdoğan said, and urged the U.N. to send personnel to carry out the necessary inspections.He continued by saying that it is evident Russia is not fighting nor does it intend to fight Daesh in Syria."The Syrian crisis is going into deadlock due to Russia's intervention," Erdoğan said."There is no such thing as a good terrorist or a bad terrorist," he added, and urged the U.S. once again to decide whether it is on Turkey's side or the PYD's.He criticized the U.S. for refraining from displaying their stance, while he underscored that Turkey will continue to respond in retaliation to mortars.Erdoğan also said that the Assad regime is only fighting the moderate opposition, and is not fighting Daesh, contrary to their claims."We have come to the end of the road. Either this issue will be solved or it will capture the whole world and lead to even bigger problems," the president said, noting that the Syrian crisis could have a snowball effect and have global implications.Russian-backed Assad forces have advanced towards the Turkish border in a major offensive in recent weeks. YPG fighters, regarded by Ankara as terrorists, have taken advantage of the violence to seize territory from Syrian rebels, as Turkey accuses the YPG of pursuing "demographic change" in northern Syria by forcibly displacing Turkmen and Arab communities.Ankara has been requesting that the U.N. establish "safe zones" in Syria since 2012 but could not find the necessary international support.According to Reuters, German Chancellor Angela Merkel on Wednesday also called for a no-fly zone to protect civilians in Syria, which she said would also help the political process.Turkey, 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, avoiding the need to bring them into Turkey.