Syria has been in a state of civil war for almost three years.
A car bomb in northern Syria killed at least 26 people on Wednesday, most of them rebels, a monitoring group said.
The bomb struck the Aleppo province town of Jarablos, scene of fierce fighting in recent days between the jihadist Islamic State of Iraq and the Levant, and rival rebels.
Among the dead were three civilians, the Syrian Observatory for Human Rights said.
Observatory director Rami Abdel Rahman told AFP it was likely the incident "was a suicide attack staged by ISIL," adding that it came after jihadists lost turf to rival rebels in the town.
Activists in Aleppo province said there had actually been two car bomb attacks within minutes of each other in Jarablos.
"Two car bomb attacks struck Jarablos, one near the agriculture school, the other near the prison," said Nazeer al-Khatib, a citizen journalist with grassroots network Shahba Press.
Both sites were being used by rebels fighting ISIL as bases, said Khatib.
Previously under ISIL control, Jarablos has come under a heavy offensive by moderate and Islamist rebels since Monday night.
It is important because it is located on the Turkish border, with a frontier post.
Since January 3, the Islamic Front, the Syrian Revolutionaries' Front and the Mujahideen Army have waged a fierce offensive against ISIL in several areas of northern Syria.
While local rebels have advanced against ISIL in Aleppo and Idlib provinces, the jihadists have recaptured total control of Raqa province to the east.
On January 6, ISIL commander Abul Baraa warned rebels the group would stage suicide attacks against them should they not cease their offensive. He was killed earlier on Wednesday in a rebel attack in Idlib province.