Numerous inhabitants of the predominately Arab area that is expected to comprise the safe zone - including Tel Abyad, Ras al Ain and several Arab villages - had fled to Turkey fearing reprisals by the YPG on charges they had links to Islamic State. The zone is reportedly planned to be about 19 miles (30 km) deep. "They went from Tell Abyad to Hasheesha", said Ryad Abu Khalaf, Commander of the Syrian Democratic Forces (SDF)-affiliated Military Council in Tell Abyad.
While Turkey aims to completely eliminate the terrorist organisation nesting in the northern Syria, the United States counts for dealing with Turkey and the terrorist organisation on same ground, Erdogan said.
A Reuters witness said vehicles with Turkish flags joined those in Syria with U.S. flags some 15km east of the Turkish border town of Akcakale, near Syria's Tel Abyad.
Rather than calling it a safe zone, Washington and the Kurdish-led forces have said a "security mechanism" is taking shape to diffuse tensions in northeastern Syria.
Caggins said the patrol demonstrated the coalition's "continued 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 ISIS.
After intensive negotiations, Turkey and the United States have also set up a joint operation centre but have so far disagreed over the zone's depth and over the command structure of the forces to operate there.
Turkey, which has carried out several incursions into Syria in the course of the country's civil war in an effort to curb the expanding influence of the Kurdish forces, carried out joint patrols with US troops in the northern town of Manbij previous year, along the border of the areas controlled by Kurdish-led forces.
The patrol ended after two and a half hours, with four stops along the way in villages near the border to inspect bases.
Helicopters flew low. Local commuters patiently waited while the convoy blocked traffic.
"We don't know what this will do".
Turkish officers accompanied by US troops documented some of the YPG fortifications that had been demolished as part of an agreement that Ankara and Washington hashed out in recent months, a Reuters witness said. "We will see", said one onlooker.
"If de facto formation of a safe zone east of the Euphrates River with Turkish soldiers is not initiated by the end of September, Turkey has no choice but to set out on its own", Recep Tayyip Erdogan said during a mass opening ceremony in eastern Malatya province.
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 in every sense of the word".
Ankara hopes the safe zone will smooth the way for the return of some of the more than 3.6 million Syrian refugees living in Turkey, but experts say the situation remains unclear.
Turkey is also involved in Syria's northwest Idlib region where its troops and observation posts have come under pressure as Russian-backed Syrian government forces have pushed north in recent months.
The Turkish defence ministry said Sunday the joint patrols were "ongoing as planned without any delay to destroy terrorist shelters, to collect their heavy weapons, (and) to ensure necessary conditions for withdrawal and the future return of our Syrians brothers and sisters to the area".
Associated Press writer Suzan Fraser contributed from Ankara, Turkey.