Ankara blamed the Kurdish YPG militia for a car bomb that killed more than a dozen people and injured 30 on Saturday in a Syrian border town that Turkish-backed forces seized last month.
The Syrian Observatory for Human Rights said pro-Turkey fighters and civilians were among the dead and injured in Tel Abyad.
Turkey's state-owned Anadolu Agency said 13 were killed after a "bomb-laden vehicle" exploded.
Tel Abyad is one of two major border towns in which the heaviest fighting took place when Ankara launched the incursion on October 9 against the YPG, drawing international condemnation.
The militia had for years been allied with the US in the fight against ISIS.
The explosion comes after two weeks of relative calm in north-east Syria, and a day after Turkish and Russian troops began joint ground patrols under a deal that pushed the YPG from Turkey's border.
Moscow said the YPG had withdrawn to at least 30 kilometres from the border under the deal, but Ankara has been sceptical and said attacks could resume if the miltia remains.
"We condemn this inhuman attack of the bloody PKK/YPG terrorists who attacked the innocent civilians of Tel Abyad, who returned to their homes and lands as a result of the Operation Peace Spring," Turkey's Defence Ministry said.
Turkey's presidential spokesman, Ibrahim Kalin, also blamed the YPG.
A spokesman for the Syrian Democratic Forces, which includes the YPG, was not available for comment.
The PKK, based in Turkey, is designated a terrorist group by Ankara and its western allies.
Turkey considers the YPG to be a terrorist group because of its ties to PKK militants in south-east Turkey.
Days after US President Donald Trump's abrupt decision on October 6 to pull American troops out of north-east Syria, Turkey and allied Syrian rebels launched a cross-border offensive and seized control of Tel Abyad and land along the frontier.
Ceasefire deals Ankara made first with Washington and then with Moscow halted fighting in recent weeks.
The Observatory said about 300,000 people have been displaced by the offensive and 120 civilians killed.
The incursion, which was condemned by scores of countries in the West and the Middle East, left the Turkey-backed rebel Syrian National Army largely in control of Tel Abyad.
On Wednesday, President Recep Tayyip Erdogan said Turkey had information that the YPG had not completed its pull-out from the border region.
Russia is the Syrian government's most powerful ally and helped it to turn the tables in the country's eight and half year civil war by retaking much of the country from rebels since 2015.
The Turkish-Russian dealallowed Syrian government forces to move back into border regions from which they had been absent for years.
