(CNN) The top US envoy in northern Syria condemned the Trump administration for not working harder to keep Turkish troops from invading after US forces withdrew from the region and accused Turkey of having committed "war crimes and ethnic cleansing" there, the New York Times reported Thursday.

The paper reported that diplomat William Roebuck made the comments in a 3,200-word internal memo last month, where he weighed whether a combination of more hardline policy, economic threats and military presence could have prevented the attack, since that kind of approach has dissuaded Turkey in the past.

"It's a tough call, and the answer is probably not," Roebuck wrote in the memo, obtained by the Times. "But we won't know because we didn't try."

The Times posted the full memo Thursday afternoon. Roebuck's office declined to comment to CNN. Two administration officials confirmed to CNN that the memo was real and that the Times report is accurate.

Concerns about ethnic cleansing

While the State Department would not confirm or deny internal communications, spokesperson Morgan Ortagus said the US had "strongly disagreed" with President Recep Tayyip Erdogan's decision to attack and has ongoing concerns about reports of ethnic cleansing.

Despite the worry that Turkey's proxies may be committing war crimes and despite reports that Turkey, a NATO ally, may have violated terms dictating the use of US-provided weapons, President Donald Trump is set to host the Turkish leader in Washington on November 13, tweeting Wednesday that he looks forward to it.

Ortagus said that "in the initial days, we had serious concerns regarding reports that the Turkish-Supported Opposition may have engaged in violations of the law of armed conflict in northeast Syria, including reports of the killing of unarmed civilians and prisoners and reports of ethnic cleansing."

"Those concerns remain," she added.

She praised efforts by Vice President Mike Pence to negotiate a halt to the Turkish offensive that limited the Turks' presence "to a small roughly 80 by 20 mile box in a largely ethnic Arab area."

"We have not seen widespread ethnic cleansing there," she added.

Roebuck's memo was published as the US investigates whether Turkey violated agreements with Washington about the use of US-provided weapons and equipment and whether Ankara improperly transferred US-supplied weapons to its proxies in Syria.

CNN previously reported that the commander of the Syrian Democratic Forces had met with Roebuck shortly after President Donald Trump's decision to pull troops from the border, saying the US failure to stop Turkey's invasion had left the Syrian Kurds to be slaughtered.

Roebuck echoed criticism from lawmakers, analysts and US allies in his memo, saying that political and military turbulence had undermined American strategy in the region, the Times reported.

The precipitous withdrawal, which took allies by surprise, gave US strategic opponents Russia and Iran a stronger foothold in the region, gifted Syria large territorial gains and damaged US credibility globally. Many saw Trump's acquiescence to Erdogan as a betrayal of Kurdish allies who helped the US fight ISIS and were instrumental in tracking down ISIS leader Abu Bakr al-Baghdadi, but the damage is broader.

In an interview published Thursday, French President Emmanuel Macron told the Economist that the US shows signs of "turning its back" on Europe and can no longer be relied upon, and pointed to the unexpected US troop withdrawal from northeastern Syria last month as a stark example.

The French military has worked with US forces in the anti-ISIS fight, but may now have to leave if the US pull back means an end to American military support for their efforts.

While the aftermath of America's withdrawal was a "sideshow" compared to the bloodshed of the region's longstanding conflict, Roebuck wrote, "it is a catastrophic sideshow and it is to a significant degree of our making."

'Why didn't officials do more'

"One day when the diplomatic history is written," the diplomat wrote, "people will wonder what happened here and why officials didn't do more to stop it or at least speak out more forcefully to blame Turkey for its behavior: an unprovoked military operation that has killed some 200 civilians, left well over 100,000 people (and counting) newly displaced and homeless because of its military operation."

"To protect our interests, we need to speak out more forcefully, publicly and privately, to reduce the blame placed on the U.S. and to highlight the Turkish responsibilities for civilian well-being," Roebuck added.

The US investigation into Turkey's potential arms transfer will include looking to see whether they involved groups that some US officials say may have committed war crimes as part of the Turkish-led incursion targeting America's Kurdish allies. A US defense official told CNN that the US government currently believes the allegations that Turkey has violated end-use monitoring agreements are "credible," prompting the review.