“Will devastate Turkey economically if they hit Kurds,” Mr. Trump said on Twitter, suggesting that there would be a 20-mile safe zone around the group after American forces leave. He added, “Likewise, do not want the Kurds to provoke Turkey.”

Mr. Trump’s tweets marked the first public threat toward Turkey, a NATO ally, over the Kurds and seemed to offer a blanket of protection for the group, a band of American-backed militias that the Turkish government sees as terrorists.

Mr. Trump’s announced pullout from Syria without guaranteeing safety for the Kurds, who have helped American forces fight the Islamic State, had received sharp criticism from lawmakers. But his tweets Sunday threatened to upend Secretary of State Mike Pompeo’s attempt to reach a deal with Turkey to protect them, something Mr. Pompeo had been optimistic about when talking to reporters in Abu Dhabi, United Arab Emirates, earlier Sunday.