Armed Turkish military vehicles have crossed into northeast Syria to begin joint patrols with the United States Army as part of a planned so-called "safe zone" along the border.
Turkey’s state-run Anadolu Agency said that six Turkish-flagged armoured vehicles on Sunday joined the US military convoy from Akcakale district of southeast Sanliurfa province in Turkey.
The joint ground patrols were launched as part of the first phase of the "safe zone" plan agreed between the two countries last month, the agency added.
The AFP news agency said that two helicopters overflew the area as the Turkish vehicles drove through an opening in the concrete wall erected between the two countries.
The Turkish Defence Ministry confirmed the start of the joint patrols and said unmanned aerial vehicles were also being used.
The US-Turkey deal for the zone is intended to manage tensions between Ankara and US-backed Kurdish-led forces, who mainly control the region east of the Euphrates river and are branded by Ankara brands as "terrorists".
The details of the plan are still unclear, as is the size of the zone and how it will be managed or divided. A US-Turkish joint centre of operations was recently established as part of the agreement.
The Kurdish administration in northeast Syria said late last month that the Kurdish People's Protection Units (YPG) was pulling forces and heavy weapons from the rural areas of the region in line with the plan.
The Syrian government condemned Sunday's development "in the strongest terms", calling it a violation of international law.
"This step is an aggression in the full sense of the word and aims to complicate and prolong the crisis in Syria," a Syrian official was quoted as saying by the official SANA news agency.
Al Jazeera's Jamal Elshayyal, reporting from Hatay on the Turkey-Syria border, said the patrols were limited in terms of their reach.
"These patrols reach only the first few kilometres on the Syrian side of the border. Turkey wants to go much deeper, up to 30km into Syria, but the Americans resist this," he said.
Washington and Ankara have been at odds over the future of the northeast Syria region, where the YPG formed the main part of a US-backed force fighting the Islamic State of Iraq and the Levant (ISIL or ISIS) before the group's territorial defeat there.
Turkey in recent years has already sent its military into northern Syria twice - in 2016 and 2018 - to push Kurdish fighters from its borders, and it currently controls a region in northwest Syria together with allied rebels.
Having announced plans to withdraw US special forces from north Syria, US President Donald Trump initially proposed the zone last year, aimed at dissuading Turkey from carrying out a cross-border attack.
But Trump later suspended the plan to ensure the Kurds would be protected - until last month when Ankara and Washington reached a framework agreement.