The incident comes as the Turkish military launches air and artillery strikes across the border in an operation the United States strongly opposes.

President Donald Trump ordered the withdrawal of special operations teams from two other border outposts earlier this week to keep them out of the path of the Turkish offensive, which is targeting the same Kurdish militias that have fought alongside American troops against the Islamic State but that Turkey considers a terrorist threat.

In a call with Turkish Defense Minister Hulusi Akar yesterday, Esper “reiterated his strong concern that, despite U.S. force protection measures, Turkey's actions could harm U.S. personnel in Syria,” Pentagon spokesperson Jonathan Hoffman said in a statement earlier Friday.

"The United States remains opposed to the Turkish military move into Syria and especially objects to Turkish operations outside the Security Mechanism zone and in areas where the Turks know U.S. forces are present," DeWalt added in the new statement. "The U.S. demands that Turkey avoid actions that could result in immediate defensive action."

Earlier Friday, Joint Chiefs Chairman Gen. Mark Milley, describing the measures the Pentagon is taking to prevent Turkish fire from accidentally striking U.S. troops, also told reporters that "the Turkish military is fully aware down to explicit grid-coordinate detail of the locations of U.S. forces."