"Our interests in Syria in many fundamental ways really diverge," said former US Envoy to the international anti-ISIS coalition Brett McGurk.
He was interviewed by Chief International Anchor for CNN Christiane Amanpour on Monday.
McGurk continued: "And when President Erdogan puts on the table proposals that might look good in concept, every time we send our best people, our best planners, to really dig in to what we can actually do together, it never really pans out."
When Barack Obama was US President, Washington and Ankara both had subvert and overt programs to oust Syrian President Bashar al-Assad. The United States shifted to an ISIS-centric approach and abruptly ended its covert program against Assad after Donald Trump became president in 2016.
When ISIS quickly exploited the complexities in war-ridden Syria in 2014, McGurk began forming the international coalition of which Turkey was not a party to.
The coalition supported the establishment of Syrian Democratic Forces (SDF) which are primarily comprised of the predominately-Kurdish Peoples' Protection Units (YPG). Ankara refuses to work with the SDF.
"The opposition groups that Turkey supports that you for example would send into a safe zone are simply not groups the United States have currently worked with. They are very closely tied with extremist groups," said McGurk.
McGurk, who has worked in the Middle East since the US invasion of Iraq, abruptly left the Department of State in December, following Trump's announcement that US forces will withdrawal.
"All the border crossings are controlled by al-Qaeda. It's a very serious problem," he added.
He called Turkey a "key partner" as the coalition was being formed in mid to late 2014.
"Plan A was to work with Turkey to get a handle on this thing," McGurk argued.
Seventy-five nations are now in the anti-ISIS coalition, but not Turkey.
"I probably spent most of my time in our first year on the job, including when I was working with Gen. Allen, in Ankara because most of the material coming to fuel the ISIS war machine frankly was coming across the border from Turkey to Syria," said McGurk.
He found it disappointing that Ankara and Washington can't work more closely in the fight against ISIS.
"Quite frankly, it was disappointing because Turkey did not take much action on the border. So we've worked very hard with Turkey in various ways and nothing has really worked out," McGurk said.
The former diplomat revealed that when he and outgoing US Defense Secretary James Mattis met with coalition partners in early December there was "a unanimous view that ISIS is not defeated, this is not over."
"There's a very serious threat to Iraq," said McGurk.
The US-led coalition went into Syria without a direct UN mandate on the basis that action in eastern Syria was neccessitated to ensure the sovereignty of Iraq as extremists captured Iraq's second-largest city, Mosul.
He reiterated the goal was always an "enduring defeat" and not just beating the so-called physical caliphate.
"I don't think there would be a single analyst who would walk into the Oval Office and tell the president that this is over.
Turkey, Iran, and Russia have worked together to find a political solution outside of UN tracks. Mocow and Washington have kept de-confliction and de-escalation lines open, and Russian President Vladimir Putin has influence over Erdogan.
"We were setting up conditions to have this very serious negotiation with the Russians, which I believe was setting up in a pretty good spot," said McGurk, blaming Trump for "throwing away our leverage by leaving."
He implored Trump to reconsider orders to withdrawal from Syria.
On Wednesday, SDF members were killed in an ISIS-claimed suicide attack on a US-local force convoy in northeastern Hasaka governorate. Differing reports put the SDF killed at 4-5.
Four Americans were killed in an ISIS-claimed suicide attack in the northern city of Manbij on Wednesday.
