A senior US State Department envoy has written a highly critical assessment of the Trump administration’s abrupt withdrawal of troops from north-east Syria, US officials said on Thursday. The internal memo by the top US diplomat to northern Syria, William Roebuck, takes the administration to task for not doing enough to prevent Turkey’s invasion of the region and protect US-allied Kurdish fighters there, officials said. Turkey invaded days after US President Donald Trump ordered the small number of special forces in the area to leave, ostensibly so they would not be caught in a crossfire between the Turks and Kurds. An official described the memo, obtained by<em> The New York Times</em>, as "lengthy and harsh." In the memo, Mr Roebuck said there was no way to know if more pressure on Turkey’s president, Recep Tayyip Erdogan, would have stopped the operation. “It’s a tough call and the answer is probably not. But we won’t know because we didn’t try,” Mr Roebuck wrote. A top deputy to US special envoy for Syria James Jeffrey, he said the pullout of US forces had badly, if not irreparably, damaged the trust of the Kurds they had been fighting alongside against ISIS. He also raised concerns about the possibility that Turkish-backed militias taking part in the operation were undisciplined and could commit war crimes. Such allegations have already been made. “Turkey’s military operation in northern Syria, led by armed Islamist groups on its payroll, represents an intention-laced effort at ethnic cleansing,” Mr Roebuck was quoted as writing. He said such abuses “can only be described as war crimes and ethnic cleansing". The State Department declined to confirm or deny the existence of the memo, but offered a long statement defending the administration’s actions that tacitly admitted there was robust internal debate on Syria policy. “This administration’s job is to do what is best for US national security and the American people,” spokeswoman Morgan Ortagus said. “That is what we have done in Syria and what we will continue to do.”