MARDIN, Turkey — Turkey has handed over 18 Syrian government soldiers captured this week in northeastern Syria in a deal negotiated by Russian forces, defusing growing tensions with the administration of President Bashar al-Assad of Syria.

The Turkish Defense Ministry, which announced the handover on Twitter on Friday morning, did not say who took custody of the soldiers. They had been captured by Turkish-backed Syrian fighters south of the strategic town of Ras al-Ayn, Syria, on the limits of territory that Turkey has seized over the past three weeks to use as a buffer zone against Kurdish forces.

The announcement came as Russian and Turkish forces began their first joint patrol along the Turkish-Syrian border under an agreement to ensure the withdrawal of Kurdish forces that Turkey regards as a security threat.

Turkey controls one section of the border, while Russian and Syrian government forces have moved into other areas vacated by American troops after President Trump announced their departure last month.

Mr. al-Assad said on Thursday that his government’s ultimate goal was to restore state authority over Kurdish areas of Syria after the departure of American forces, but he added that he expected it would happen gradually.

Russia has become the main arbiter in northeastern Syria after President Vladimir V. Putin agreed to terms for policing the region with President Recep Tayyip Erdogan of Turkey on Oct. 22. Under the deal, Turkey will control a safe zone of roughly 2,000 square miles, and Russia will ensure the withdrawal of Kurdish forces from the remaining border areas and conduct joint patrols with Turkey for verification.

In an interview on state television, Mr. al-Assad described the deal between Russia and Turkey on policing the border as “temporary” and a “positive” step that would help Damascus achieve its goal.

“It might not achieve everything,” he said. “It paves the road to liberate this area in the near future, we hope.”

Syrian government forces have entered areas of northeastern Syria for the first time in years, after the departure of American forces, but Damascus has relied on the Kurdish militia, the Syrian Democratic Force and Russian troops for security. The Kurdish-led militia turned to the Assad government after its principal backer, the United States, announced its withdrawal.

Mr. al-Assad said he did not see Turkey as an enemy. He also said that Mr. Trump’s decision to keep some American troops in southwestern Syria to maintain control of the oil fields in the area proved that the United States was a colonial power. Syria did not have the means to end the American presence soon, he said, but he predicted that one day they would have to leave.

In their first patrol Friday, Russian troops met up with Turkish troops at the Syrian border town of Darbasiyah at midday. Turkish military vehicles entered Syria and the two forces drove in a joint convoy east along the border.