Doctors from Turkey's large cities have been stationed in two southern provinces to prepare for a possible incursion into northern Syria, two security sources told Reuters on Friday, adding leave for doctors in the region had been cancelled.Turkey and the United States have been working to establish a safe zone in northern Syria along the Turkish border. President Tayyip Erdogan has said that Washington is stalling the process and that Turkey would carry out an operation into Syria if the safe zone is not set up by the end of September. A security source said doctors from some large cities were stationed in the southern provinces of Sanliurfa and Mardin in preparation for a possible incursion."President Erdogan clearly spoke of Turkey's concern over PYD/YPG presence in Syria and pointed to a possible military operation if there is no developments in the second half of September," the source said, referring to the Syrian-Kurdish militia that controls most of northern and northwestern Syria.The YPG has been a main U.S. ally in the fight against Islamic State in Syria. U.S. support for the fighters has infuriated Turkey, which considers the YPG a terrorist organization, and strained ties with Washington.Next week, Erdogan and U.S. President Donald Trump will discuss plans for the safe zone along 450 km (280 miles) of Syrian border stretching east from the Euphrates river to the Iraqi border.Turkey has already carried out two incursions against the YPG in Syria."The leaves of doctors have been suspended to prepare for a possible cross-border operation. We have been preparing for a long time. Now it is at a phase that the operation can be conducted whenever deemed necessary," a senior security official said.