The Islamic State group has claimed a suicide attack on its former Syrian stronghold of Raqa, heightening Kurdish fears of a jihadist resurgence if Turkey goes ahead with a threatened invasion.
Overnight two IS fighters armed with guns, grenades and explosive belts launched a "violent attack" on security forces in Raqa, according to the Syrian Observatory for Human Rights.
Ensuing clashes between the assailants and fighters affiliated with the Kurdish-led Syrian Democratic Forces (SDF) lasted more than one hour, the Britain-based war monitor said.
IS claimed responsibility for the assault on the Telegram messaging application.
The SDF, which runs most of northeast Syria and has been a key US ally in the fight against the jihadists, warned the assault is an "initial repercussion" of a feared invasion by Ankara.
IS has regularly orchestrated attacks in Kurdish-held territory since the SDF declared the territorial defeat of the group earlier this year.
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The SDF has warned that the IS threat may grow if it redirects its attention to defending against a Turkish offensive which Ankara said Tuesday would start "shortly" against Kurdish militias it considers terrorists.
The attack in Raqa ended when the two militants detonated their explosive belts after being surrounded, according to the Observatory, which relies on a network of sources inside Syria.
There were no immediate reports of casualties among security forces.
Kurdish forces took heavy losses in the US-backed campaign against IS in Syria which they spearheaded.
The SDF on Monday warned that a Turkish offensive could enable the IS's surviving leaders to come out of hiding.
It also said that IS cells would break out detained jihadists from Kurdish prisons and take over camps where their relatives are held.
But Turkey's presidential spokesman Ibrahim Kalin said Monday that Ankara would continue to fight IS "and will not allow it to return in any shape and form.