"To be frank, there is some concern given some of the drift toward authoritarianism," Scaparrotti told the US House of Representatives Armed Services Committee.
"That’s going well, but I do have concerns," he added.
Voters in Turkey will decide next month whether to approve a proposal to give President Recep Tayyip Erdogan sweeping new powers as president.
Since thwarting a coup attempt last July, Turkey has arrested thousands of people, including military officers, high-ranking civil servants, teachers and journalists.
US Secretary of State Rex Tillerson is scheduled to meet Erdogan later this week in Ankara.