US President Donald Trump appeared to bow to Turkish demands when he reportedly told his counterpart Recep Tayyip Erdogan that the Pentagon would stop arming Syrian Kurds, Washington’s closest ally in the fight against the Islamic State, but observers said the move could undermine efforts to stabilize the war-torn country.

The United States’ cooperation with the People’s Protection Units (YPG) since at least 2014 has roiled ties with NATO partner Turkey, which says the militant group is controlled by the Kurdistan Workers Party (PKK), listed as a terrorist organization for its three-decade armed campaign against the Turkish state.

Trump’s surprise pledge came as the US-led coalition continues to battle IS in pockets of Syria and raised concerns about the post-conflict period, when the Kurds are expected to play a central role in rebuilding shattered cities, promoting civilian governance and preventing Syrian President Bashar al-Assad from retaking control of areas they now run. It will also further erode Washington’s clout in a region it appears to be ceding to rivals Iran and Russia, analysts said.

Trump tweeted Nov. 24 that he would speak with Erdogan “about bringing peace to the mess that I inherited in the Middle East.” After the men spoke, Foreign Minister Mevlut Cavusoglu, who was present for the call, told reporters Trump said the United States would no longer supply arms to the YPG-led Syrian Democratic Forces, six months after it announced it was sending them heavy weapons. “He clearly said that this nonsense should have been ended sooner,” Cavusoglu stated.