Turkish President Recep Tayyip Erdogan threatened on Sunday to carry out an operation east of the Euphrates river in northern Syria, in an area controlled by Kurdish forces. Turkey has been running out of patience with the United States, which made an agreement with Ankara to implement a safe zone in northeastern Syria. Erdogan said both Russia and the United States have been told of the operation. Following US President Donald Trumps announcement last year of a planned US withdrawal from northern Syria, the two NATO allies agreed to create a safe zone inside Syria along its northeastern border with Turkey, that would be cleared of the YPG Kurds. The YPG was Washingtons main ally on the ground in Syria during the battle against ISIS, but Turkey sees it as a terrorist organization. Ankara says that the United States has stalled progress on setting up the safe zone and has demanded that Washington sever its relations with the YPG. The operation, which would mark the third Turkish incursion into Syria in as many years, was first signaled by Erdogan earlier this year but later put on hold. In an apparent message to the US-allied Kurdish fighters in Syria, Erdogan told party officials on Sunday that "those who engage in bullying by putting their trust in foreign forces will tomorrow find themselves in the grave." "We entered Afrin, Jarablus, and Al-Bab. Now we will enter the east of the Euphrates," Erdogan said during a motorway-opening ceremony. "We shared this with Russia and the United State,” he added. Since 2016, Turkey has launched two cross-border offensives against ISIS and the Syrian Kurdish fighters.