The Middle East appears on the brink of wider sectarian war engulfing Iraq, with radical Islamist insurgents wantonly kidnapping, torturing and killing civilians, UN human rights investigators said in a report on Tuesday.
Militants of the Islamic State of Iraq and the Levant (ISIL) group have routed Baghdad's army and seized the north of Iraq in the past week, linking it with a major swathe of territory previously taken in eastern Syria during the civil war there.
"We predicted a long time ago the dangers of spillover both ways, which is now becoming a regional spillover," said Vitit Muntarbhorn, an international law expert who took part in the inquiry. "We are possibly on the cusp of a regional war and that is something we're very concerned about."
UN human rights Navi Pillay said on Monday forces allied with ISIL in northern Iraq had almost certainly committed war crimes by executing hundreds of non-combatant men over the past five days.
A report presented on Tuesday to the UN Human Rights Council said foreign Sunni jihadi militants and funds had poured into Syria where rebel factions including ISIL were wantonly abusing civilians in zones they controlled.
"A regional war in the Middle East draws ever closer. Events in neighbouring Iraq will have violent repercussions for Syria," the investigators' report said.
"Growing numbers of radical fighters are targeting not only Sunni (Muslim) communities under their control but also minority communities including the Shi'ites, Alawites, Christians, Armenians, Druze and Kurds," it said of Syria.
Its reference to Sunni militants targeting Sunni civilians involved forceful pressure on Sunni women to comply with sharia (Islamic religious law) and acts of revenge against Sunnis who had served in the Syrian government.
Syrian President Bashar Al Assad's minority Alawite sect, an offshoot of Shi'ite Islam, has dominated Syria for decades.
The report said ISIL kidnapped nearly 200 Kurdish civilians in an attack in the Syrian city of Aleppo at the end of May.
Muntarbhorn said ISIL was fighting other rebel groups in Syria more than it was fighting the Damascus government.
"ISIL has shown itself willing to fan the flames of sectarianism, both in Iraq and in Syria. Any strengthening of their position gives rise to great concern," the report said.
At least 160,000 people have been killed in Syria's three-year-old conflict that began as peaceful demonstrations against President Bashar al-Assad and escalated into civil war.
On Iraq, US President Barack Obama was considering options for military action to support Baghdad's besieged government but has made no decision on the US response to the ISIL onslaught threatening to tear apart the country.
Covering the period since mid-March, the UN report described how Syria's government had regained strategic terrain in Damascus and Aleppo provinces through a mix of "brutal tactics with long-lasting sieges", marked by barrel bombing.
"Victims describe...the agony of being encircled, shelled and bombarded while slowly starving," the Brazilian chief investigator, Paulo Pinheiro, told reporters in Geneva.
The team of about 20 UN investigators has interviewed 3,000 Syrians in Syria or neighbouring countries via Skype.
They say they have documented war crimes and crimes against humanity committed by all sides and have drawn up four confidential lists of suspects for future prosecution.
"The international community, and specifically the (UN) Security Council, have yet to demand that the individuals perpetrating crimes against the men, women and children of Syria are held responsible. Through their inaction, a space has been created for the worst of humanity to express itself," the report said.
The inquiry has received thousands of photos published in January, smuggled out by a former Syrian military police photographer who said the images showed people tortured and killed in government-run detention centres.
"Many are emaciated, almost all bear marks of horrific abuse, including strangulation, mutilation, open wounds, burns and bruising," Pinheiro said. "Such injuries are consistent with torture methods previously documented by the commission in our 12 reports until now."
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