The New York Times said it had obtained an internal memo written by William Robak, the US deputy special envoy to the coalition against ISIS, in which he noted that the United States "did not try" to take stronger measures to curb Turkish President Recep Tayyip Erdogan.
He criticized the "tireless efforts of ethnic cleansing" by Turkey and its allies against the Kurds in Syria, which "can only be defined as war crimes or ethnic cleansing."
"One day, when diplomatic history is written, one will wonder what happened here and why officials did not do more to prevent this, or at least (why they did not) speak more forcefully to blame Turkey for its behavior," the paper said.
State Department spokeswoman Morgan Ortigas declined to say whether these "alleged internal private contacts" were correct. "We have made it clear that we strongly disagree with Erdogan's decision to enter Syria, and we have done everything except the military confrontation to prevent this," she said.
The spokeswoman said the United States had taken seriously reports that Turkish-backed fighters had committed abuses, including the killing of civilians.
"These questions remain, and we have raised the issue at the highest levels in the Turkish state," she said.