Turkey’s attack on Kurdish militants east of the Euphrates River in northern Syria for the first time escalates its fight against the US-allied group and raises the risk of a confrontation with American forces fighting alongside the rebels.

The shelling of People’s Protection Units (YPG) positions on the Euphrates’ eastern shore on Sunday came just two days after President Recep Tayyip Erdogan gave his “final warning” to Syrian Kurdish fighters to retreat and accused the United States of dragging its feet in an agreement to remove the group from Turkey’s border.

The attack also followed a summit on Syria in Istanbul, hosted by Erdogan and attended by the leaders of Germany, France and Russia on Saturday. They called for a new Syrian constitution to be drafted before the end of this year, “paving the way for free and fair elections,” according to a joint statement.

Turkey’s active role on Syria appears to be part of Erdogan’s wider strategy to reassert Turkish influence in the region. Nowhere is this more apparent than in the Turkish president’s high-stakes standoff with Saudi Arabia’s crown prince, Mohammed bin Salman, over the case of murdered journalist Jamal Khashoggi, a Washington Post columnist who was killed at the Saudi Consulate in Istanbul Oct. 2.