The armies of Syria and Turkey traded deadly fire Tuesday for the first time since Ankara launched an anti-Kurdish offensive in early October, as Russia announced Kurdish forces had withdrawn from the border area.
Russian Defense Minister Sergei Shoigu said Kurdish forces had pulled back from the entire border as per a deal struck between Ankara and regime backer Moscow in Sochi earlier this month.
The Turkish military and its Syrian proxies launched an offensive against Kurdish forces in northeastern Syria on October 9 with the aim of creating a buffer zone roughly 30 kilometers (20 miles) deep.
"The withdrawal of armed units from territory where a security corridor should be created has been completed ahead of time," Shoigu said, as quoted by Russian news agencies.
He added that Syrian border guards and Russian military police had been deployed in the area.
Earlier this month, Kurdish forces agreed to withdraw from a 120-kilometer long segment of the 440-kilometer border zone, but clashes have been reported since.
The Turkish presidency said joint Turkish-Russian patrols -- also planned under the Sochi deal -- would verify the Kurdish forces' withdrawal.
But the situation was complicated by clashes between Syrian and Turkish forces on Tuesday.
The Britain-based Syrian Observatory for Human Rights reported that "heavy fighting erupted for the first time between the Syrian and Turkish armies", adding that six Syrian soldiers were killed near the key border town of Ras al-Ain.
"Turkish artillery fire killed five regime forces in battles on the edge of the village of Assadiya," Rami Abdel Rahman, the head of the Observatory, told AFP.
He added that Syrian fighters used by Turkey as the main ground force for the invasion had executed a regime soldier they had captured.
Left in the lurch after US troops withdrew from the border area, Kurdish forces turned to the Syrian regime for protection.
The regime's forces moved quickly north and are now expected to deploy along much of the border zone.
Turkish-Russian patrols in a 10-kilometer-deep strip were to start on Tuesday at 1600 GMT, but strikes near the border town of Derbasiyeh threatened that deadline, both the Observatory and Syrian state media reported.
The Observatory said Turkish and Russian military units had been due to meet at a border crossing to discuss the upcoming patrols.
But on Tuesday, Syrian state news agency SANA reported "Turkish mortar fire on the Derbasiyeh border crossing", some 60 kilometers east of Ras al-Ain, and said six Syrian civilians had been wounded.
It added that rounds were fired as Russian military police were driving by.
The Syrian Democratic Forces, de facto army of the moribund autonomous Kurdish administrated territory, has voiced reservations over the Sochi deal.
The agreement, to which the Kurds are not signatories, essentially hands much of their heartland to the regime.