ISIL, which has been disavowed even by Al-Qaeda, said it held the seven responsible for a grenade attack on one of its fighters earlier this month in the Euphrates Valley city of Raqa, which it rules with an iron fist.

"Ten days ago, attackers on a motorbike threw a grenade at an ISIL fighter at the Naim roundabout. A Muslim civilian had his leg blown off and a child was killed," the group said on Twitter.

"Our fighters immediately set up a roadblock and succeeded in capturing them. They were then able to detain other members of the cell."

The Britain-based Syrian Observatory for Human Rights posted a photograph of the two prisoners being crucified at the roundabout with passer-by walking past apparently unfazed.

One of them, blindfolded and with his head spattered with blood, had a banner wrapped round his body proclaiming: "This man fought against Muslims and threw a grenade in this place."

The monitoring group said they were not the first crucifixions by ISIL. On April 16, its fighters executed a man for theft from a Muslim in the same way.

ISIL's exactions caused a backlash against them from rival rebel groups, including Al-Qaeda's official Syria affiliate Al-Nusra Front, who joined forces against its fighters from the start of the year.

ISIL has now been forced out of much of northern Syria but its fighters remain entrenched in Raqa — the only provincial capital entirely outside Syrian government control — and much of its surrounding province.

The group also carried out three executions in the Raqa provincial town of Tal Abyad, on the Turkish border, and two more in the town of Saluq, the Observatory said.