President Recep Tayyip Erdoğan called U.S. Vice President Joe Biden on Monday to express condolences for his son Beau Biden, who died of brain cancer on May 30 and to discuss the ongoing crisis in Syria.According to presidential sources, the two leaders discussed the recent developments in Syria as President Erdoğan voiced Turkey's concerns over the advance of the Kurdish Democratic Union Party (PYD) in the war-torn country.On Monday, the U.S. State Department spokesman John Kirby said that he doesn't have any knowledge of the Vice President's phone conversations but commented on Turkey's plan to establish a buffer zone in Syria, by stating that the challenges Turkey faces on its southern borders are understandable but neither Washington nor a U.S.-led coalition would currently support such a buffer zone."Nobody's turning a blind eye to the challenges they're [Turks] facing or the concerns that they have," said Kirby."The Defense Department has made it clear that they don't believe that there's a need for that [buffer zone] at this time and that the use of coalition military assets in trying to affect a zone like that would entail an awful lot in terms of logistics, time, resources and effort."Ankara and Washington have been on different pages over Syria, as the U.S. wants to solely focus on fighting against theself-proclaimed Islamic State of Iraq and al-Sham (ISIS) while the Turkish government has been calling on the international community to take a step against the Bashar al-Assad regime.Turkey has also suffered from the Syrian civil war by being host to more than two million refugees, which has cost the country $5.6 billion so far.