Ten members of Islamic State were summarily executed in Syria's Idlib province on Saturday by Tahrir al-Sham, according to Ebaa News Agency, a news outlet that supports the rival jihadist alliance.
Ebaa said the executions were in response to an Islamic State suicide bomb attack at a restaurant in Idlib a day earlier.
Although Islamic State and the groups that make up Tahrir al-Sham, including Al Qaida's former affiliate the Nusra Front, subscribe to hardline jihadist ideology, they have opposed each other for years.
Islamic State has lost virtually all its territory to the Syrian army, backed by Russia, Iran and Shi'ite Muslim militias, and a rival campaign by Kurdish and Arab groups in the north supported by the United States.
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Ebaa quoted Anis al-Shami, a security official from Tahrir al-Sham, saying that the executions "today at the scene of the crime are fair punishment which might deter them and wake them from their stupor".
It published images showing masked gunmen in fatigues firing handguns at the heads of 10 bearded men who sat in front of them on the pavement. Reuters could not independently verify the agency's report.
Tahrir al-Sham is the main jihadist group in northwest Syria, with a large armed presence throughout Idlib including along the Turkish border.
Islamic State, which seized large swathes of territory in Syria and neighbouring Iraq in 2014, now faces final territorial defeat in a tiny enclave in eastern Syria.