Car bomb kills 16 including 3 Turkish personnel in NE Syria: monitor
The other 11 killed were local security forces or members of a Turkish-backed faction manning the checkpoint, the Britain-based Syrian Observatory for Human Rights said.
Twelve more were wounded, it said.
Turkey said two of its gendarmes had been killed and a further eight wounded.
Turkish forces and their Syrian proxies last year seized a 120-kilometre (75-mile) stretch of land inside the Syrian border from Kurdish forces, running from Ras al-Ain to Tal Abyad.
Such bombings are common in Ras al-Ain.
In July, the blast from an explosives-rigged motorbike ripped through a vegetable market there, killing at least eight people, including six civilians.
The Kurdish-led People's Protection Units (YPG), from whom the Turks and their allies seized the territory, played a key role in the US-backed fight against the Islamic State jihadist group in Syria.
But Ankara views them as an extension of the Kurdistan Workers' Party (PKK) that has waged a deadly insurgency in southeastern Turkey since 1984.
Syria's civil war has killed more than 387,000 people and displaced millions from their homes since erupting in 2011 with the brutal repression of anti-government protests.