President Trump threatened Turkey on Sunday with harsh economic sanctions if it attacks Kurdish forces in Syria after American troops withdraw from the country in the coming months.

“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.