The US did not give Turkey a "green light" for its offensive in northern Syria, US Secretary of State Mike Pompeo has said.
Mr Pompeo defended President Donald Trump's decision to withdraw US troops from the border area, which has sparked an outcry at home and abroad.
Turkey has now launched an assault on territory held by Kurdish-led forces.
President Recep Tayyip Erdogan said the aim was to "prevent the creation of a terror corridor" on the border.
Turkish forces plan to make a "safe zone" cleared of Kurdish militias which will also house Syrian refugees.
BBC
Russia's unmanned Luna-25 spacecraft has crashed into the Moon after spinning out of control, officials say.
Donald Trump has confirmed that he will not take part in Republican presidential debates with his rivals in the race for the White House.