The U.S. has sent about 40 special-operations troops to work with Turkish forces on clearing out a stretch of the border region in northern Syria still occupied by Islamic State, U.S. officials said.

It is the first time American special-operations forces have worked jointly with the Turkish military inside Syria, a deployment U.S. officials view as a way to strengthen the relationship with a key ally in the fight against Islamic State. The action also comes as a show of support two months after a failed coup rattled the Turkish...