The presence of Turkish troops inside Kurdish-administered areas is a major development in the conflict along the border. Over the past four years, Washington has often had to play the role of arbiter, trying to forestall violence between its NATO ally Turkey and its local Syrian partners, the Kurdish-led fighters.
So far, the Kurdish-led forces have withdrawn as deep as 14 kilometers (nine miles) from the border and have removed defensive positions, sand berms and trenches.
The depth of the zone, as well as who will control it, is still being worked out.
Several Turkish armored vehicles with the country's red flag crossed into Syria where U.S. troops were waiting for them. About half a mile away, more U.S. armored vehicles flying the American flag waited for the patrol to begin. Associated Press journalists in the area around the village of Tal Abyad Sunday saw the vehicles linking up, with the U.S. vehicles leading the 12-vehicle convoy.
With local farmers and children looking on, the convoy then drove through the rolling Syrian countryside, a patchwork of green farms and dry scrubby grassland. The joint patrol ended after two and a half hours, with four stops along the way in villages near the border.
Local commuters patiently waited while the convoy blocked traffic.
"We don't know what this will do. We will see," said one onlooker.
The Syrian government, which withdrew from the area in the chaos of war after the conflict erupted in 2011, condemned the joint patrol Sunday and labeled it "an aggression."
Even as the patrol was taking place, Turkish President Recep Tayyip Erdogan said serious differences remained with the Americans.
"Our negotiations with the United States on the safe zone are continuing but we see with every step that what we want and what is in their head is not the same thing," he said.
"It is clear that our ally is trying to create a safe zone for the terrorist organization, not for us," he added, addressing crowds in the city of Malatya, eastern Turkey.
AP reporters in Tal Abyad said the patrol inspected a couple of Kurdish-controlled bases, where only days before workers had removed sand berms and closed trenches. U.S. troops had inspected the two bases on Saturday during patrols with the local Kurdish-led forces.
An initial agreement between Washington and Ankara last month averted threats of a Turkish incursion.
But in a reflection of the precarious Washington finds itself, Erdogan appeared to criticize the process, saying Sunday that a zone cannot be achieved with "with five or 10 vehicle patrols or with the deployment of a few hundred soldiers for show."
Details of the safe zone deal are still being worked out in separate talks with Ankara and the Kurdish-led forces in Syria, known as the Syrian Democratic Forces. The force is dominated by the People's Protection Units, or YPG, considered by Ankara to be 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.
In a statement, coalition spokesman Col. Myles B. Caggins III said the Turkish forces "observed first-hand progress on destroyed YPG fortifications and areas where YPG elements voluntarily departed the area." He said the patrol demonstrates America's commitment to address Turkey's legitimate security concerns "while also allowing the Coalition and our SDF partners to remain focused on achieving the enduring defeat of Daesh," an Arabic acronym for the Islamic State group.
Turkey has carried out several incursions into Syria during the course of the country's civil war in an effort to curb the expanding influence of the Kurdish forces. U.S. and Turkish troops carried out joint patrols in the northern town of Manbij last year, along the border of the areas controlled by Kurdish-led forces.
Sunday's joint patrol was the first one taking place east of the Euphrates River, where U.S. troops have a greater presence, and as part of the so-called safe zone that is still in the making.
The Kurdish-led administration in northeast Syria said it had agreed to the joint patrols, which came as part of the agreements that the U.S.-led coalition had reached with Turkey to avert a war. However, it said it would not accept any Turkish troops' permanent presence inside areas it controls.
Associated Press writer Suzan Fraser contributed from Ankara, Turkey.