US Defense Secretary Mark Esper has warned Turkey to halt its incursion into northeastern Syria, warning it could have "serious consequences" for Ankara, the Pentagon said Friday. In a phone call on Thursday with Turkish Defense Minister Hulusi Akar, Esper reiterated that the United States opposes Turkeys assault on the mostly Kurdish region because it risks a reversal of the gains made by the coalition fighting to eliminate the ISIS group, the Pentagon said in a statement. Esper "strongly encouraged Turkey to discontinue actions in northeastern Syria," it said. "While the secretary reaffirmed we value our strategic bilateral relationship, this incursion risks serious consequences for Turkey," the statement said. Esper told Akar that halting Turkeys incursion would provide room for negotiations to "find a common way to deescalate the situation before it becomes irreparable." On Thursday Turkey began shelling positions of the Kurdish-led Syrian Democratic Forces, a key US ally in the five-year campaign to crush the ISIS group. Turkey says the group backs Turkish Kurd militants, which it labels terrorists. On Sunday President Donald Trump appeared to give a green light to a Turkish invasion, announcing that Washington would pull back several dozen US special forces fighters positioned on the Syria-Turkey frontier. Trump has emphasized that the United States finally defeated ISIS militants early this year and needs to get out of Middle East conflicts. However, Congress and the Pentagon have stressed the need to protect the Syrian Kurds from Turkey, many pointing out that 11,000 SDF fighters died in the anti-ISIS campaign. They also warn that the ISIS group could easily make a comeback if the SDF is forced to free some 10,000 ISIS prisoners while they battle Turkey.