The Donald Trump administration’s top envoy for Syria engagement, Jim Jeffrey, met with Turkish presidential spokesman Ibrahim Kalin in Ankara today to kick off talks on a proposed safe zone in northern Syria amid continued disagreement between Turkey and US-backed Syrian Kurdish military commanders over its shape, size and purpose.

The pro-government Turkish daily Sabah said the US and Turkish delegations had discussed the safe zone alongside what it described as “operations east of the Euphrates” and other Syria-related issues. Turkey has repeatedly threatened to mount a military offensive against the Syrian Kurdish People’s Protection Units (YPG) over the group's links to the Kurdistan Workers Party (PKK), the guerrilla outfit that has fought the Turkish army for Kurdish independence and then autonomy for the past 35 years.

Turkey wants a lead role in a safe zone that would be 32 kilometers (20 miles) deep and stretch the length of Syrian Kurdish-controlled territory all the way to Iraq. The commander of the YPG-led Syrian Democratic Forces (SDF), Mazlum Kobane, has reportedly agreed to a cordon sanitaire to stave off Turkish attacks that would be no more than five kilometers (three miles) deep and protected by local Arab and Kurdish forces, who would ostensibly be subject to coalition — and by extension Turkish — vetting. But Turkey isn’t interested and the YPG is dead set against letting Turkish boots onto their soil, suggesting they would only do so if they got Turkish-occupied Afrin back, a proposal they know to be a non-starter.

It remains unclear how Jeffrey hopes to bridge differences when both sides are in no mood to compromise.