"On Syria, Turkey is not a reliable partner," McGurk wrote in the Washington Post.
McGurk explained that the Syrian opposition forces Ankara supports are "marbled with extremists" and are too small in numbers to serve as an alternative to the Syrian Democratic Forces or have an effective impact in challenging Syrian President Bashar Assad.
Turkey cannot operate in Syria without substantial US military support, he added.
The former envoy also said discussions are underway to have the Turkish military or Turkish-backed opposition fighters moved into areas of northeastern Syria that are now controlled by the SDF.
McGurk's comments come after on Wednesday, nearly 30 people were killed in Manbij in an explosion claimed by the Daesh terrorist network, including two US soldiers and two American civilians.
In December, Turkish President Recep Tayyip Erdogan stated that Ankara was ready to launch a military operation against Kurdish fighters on the eastern bank of the Euphrates River as well as in Syria's Manbij, located near the Turkish border, if the United States did not remove the militia from there. He later said that he had decided to postpone the start of the military operation in Syria after a phone conversation with Trump on December 14, during which the US president also revealed his plans to withdraw troops from Syria.
Trump's directive to the Pentagon to bring American troops back home from Syria led to a shake-up in the Defense Department. Secretary of Defence James Mattis handed in his resignation shortly after the announcement, indicating that Trump's Syria policy was one of the main sources of disagreements between himself and the president. McGurk also resigned in the wake of the decision to withdraw.
*Daesh (aka Islamic state/ISIL/ISIS/IS) is a terrorist group banned in Russia