At least three of the victims killed were civilians, but the identity of the other two was not immediately clear, the Britain-based Syrian Observatory for Human Rights said.
Four others were wounded, it said.
The Turkish defence ministry said two children had been killed and two civilians wounded.
Turkish forces and their Syrian proxies in October 2019 seized a 120-kilometre (75-mile) stretch of land inside the Syrian border from Kurdish forces, running from Ras al-Ain to Tal Abyad.
Turkey blamed the attack on the Kurdish-led People's Protection Units (YPG), which Ankara accuses of being the Syrian offshoot of the outlawed Kurdistan Workers' Party (PKK), that has waged an insurgency against Turkey since 1984.
The YPG has also played a key role in the US-backed fight against the Islamic State jihadist group in Syria.
Such bombings are common in Ras al-Ain. On December 10, a car bomb killed 16 people including two civilians and three Turkish personnel at a checkpoint in the town.
In July, the blast from an explosives-rigged motorbike ripped through a vegetable market there, killing at least eight people, including six civilians.
More than 387,000 people have been killed and millions forced from their homes since Syria's civil war started in 2011 with the brutal repression of anti-government protests.
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