The Syrian government and mainstream opposition must seek a compromise to end three and half years of bloody civil war and block the rise of murderous jihadists, UN investigators said Tuesday.
"The rise of (Islamic State fighters) has emphasised the need for the government and mainstream opposition to find common ground and to commit to making compromises," said Paulo Pinheiro, who heads the United Nation's Commission of Inquiry on Human Rights in Syria.
Presenting the commission's latest report on the situation in the war-ravaged country to the UN Human Rights Council in Geneva, Pinheiro said he had grown weary of standing before world diplomats and begging for action that never came.
"We have charted the descent of the conflict into the madness where it now resides," he said, lamenting that he and his three commission colleagues had in vain "implored the parties and influential states to forge a peaceful settlement".
"This inaction has allowed the warring parties to operate with impunity and nourished the violence that has consumed Syria," he said.
"Its most recent beneficiary is ISIS," the jihadist group calling itself Islamic State (IS), Pinheiro added.
While the widespread abuses, including "war crimes" and "crimes against humanity" carried out by Syrian President Bashar al-Assad's regime and opposition groups have not subsided, the parties and the international community should find common ground in ending the "terror" delivered by IS, he said.
In its report, the commission detailed a litany of horrors committed by the group that has declared an Islamic "caliphate" in an area spanning northern Iraq and eastern Syria.
Massacres, beheading boys as young as 15 and amputations and lashings in public squares as residents, including children, are forced to watch, figure on the list, as does the widespread use of child soldiers and stoning women to death for suspected adultery.
Referring to the group's recent beheadings of two US journalists and a British aid worker, Pinheiro stressed that IS "has continued to subject scores of Syrians to the same fate in public squares in the north and east of the country".
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- 'Once a day, every day' -
An Aleppo-based journalist held in IS custody late last year told the commission how militants each day would come into cells initially crammed with around 50 people.
They blindfolded the chosen men, took them outside, and then, gunshots rang out.
"This occurred once a day, every day," he said in testimony published by the commission Tuesday, adding that one day, 19 detainees were killed from his cell at the same time.
Outrage over the group's actions spurred US President Barack Obama last week to order expanded air strikes against the militant group in Iraq and threaten the same on the Syrian side of the border.
"As military action on ISIS positions seems increasingly likely, we remind all parties that they must abide by the laws of war," Pinheiro said, demanding that "serious efforts must be made to preserve civilian life".
He also stressed that the jihadists were not "the sole agents of death and destruction" in Syria, where the UN says nearly 200,000 people have died since the conflict erupted in March 2011.
"The Syrian government remains responsible for the majority of the civilian casualties, killing and maiming scores of civilians daily, both from a distance using shelling and aerial bombardment and up close, at its checkpoints and in its interrogation rooms," Pinheiro told the Human Rights Council.
He lamented the regime's "starvation or submission" strategy through drawn-out sieges and indiscriminate attacks.
Checkpoints meanwhile "are often the starting point of a horrific journey of disappearance, torture, sexual abuse and, for many, death," he said, calling the barriers "a source of terror to the civilians they encircle".
"I have run out of words to depict the gravity of the crimes committed inside Syria," Pinheiro said.