Turkish and U.S. troops conducted their first joint ground patrol in northeastern Syria on Sunday as part of a so-called “safe zone” that Ankara has been pressing for in the volatile region.

Turkey hopes the buffer zone, which it says should be at least 30 km deep, will keep Syrian Kurdish fighters away from its border. Turkey considers these Kurdish militias a threat, but they’ve also been key U.S. allies in the fight against the Islamic State group. So far, the Kurdish-led forces have withdrawn 14 km only.

The depth of the zone, as well as who will control it, is still being worked out.

Associated Press journalists in the countryside of Tal Abyad on Sunday saw about a dozen Turkish armoured vehicles with the country’s red flag standing along the border after crossing into Syria, and American vehicles about a mile away waiting.

The two sides then came together in a joint patrol with American vehicles leading the convoy.

At least two helicopters hovered overhead at the border. The Turkish Defence Ministry confirmed the start of the joint patrols and said unmanned aerial vehicles were also being used.

The patrol inspected several Kurdish-controlled bases, apparently to ensure that trenches and sand berms had been removed. U.S. troops had inspected the base on Saturday during patrols with the local Kurdish-led forces, during which some of the berms Turkey had complained about were removed.

Washington has in the last years frequently found itself trying to forestall violence between its NATO ally Turkey and the Kurdish fighters it partnered with along the border to clear of IS militants.

An initial agreement between Washington and Ankara last month averted threats of a Turkish attack. But details of the deal are still being worked out in separate talks with Ankara and the Kurdish-led forces in Syria known as the Syrian Democratic Forces, or SDF. The force is dominated by the People’s Protection Units, or YPG, considered by Ankara a terrorist group because of its links to Kurdish insurgents inside Turkey.

While Turkish officials are calling it a “safe zone,” Washington and the Kurdish-led forces speak of a “security mechanism” taking shape to diffuse tensions in northeastern Syria.