His comments come days before Erdogan is scheduled to visit the White House and hold a joint news conference with Trump. The two NATO allies have clashed lately on issues from Turkey’s buying the S-400 system to its invading northern Syria after U.S. troops began their withdrawal there.

After outraged reactions to the invasion from lawmakers on both sides of the aisle, Trump announced sanctions on Turkey, but he removed them after a cease-fire was agreed. Even other NATO allies have chimed in, with President Emmanuel Macron of France saying on Thursday that the organization is suffering a “brain death” because of a lack of American support and resolve.

“You have partners together in the same part of the world, and you have no coordination whatsoever of strategic decision-making between the United States and its NATO allies. None,” Macron told The Economist . “Strategically and politically, we need to recognize that we have a problem.”

Still, O’Brien said “cracks … have formed” in NATO because not all members are “paying their fair share” — an issue Trump is very strict on. Asked whether Turkey committed war crimes during its military operation, O’Brien replied, “Some of the things we’ve seen are very disturbing.”

“Turkey has assured us that those are being investigated,” he said, adding: “The U.S. won’t stand by for it and — and we’ve made that position very clear to the Turks.”

In addition to Erdogan’s visit, Trump is scheduled to meet with NATO Secretary-General Jens Stoltenberg next week.