Chairman of the United Nations Commission of Inquiry on Syria, Paulo Sergio Pinheiro, who presented the commission's latest report, speaks on the situation in the war-ravaged country to the UN Human Rights Council in Geneva on Tuesday.
UN war crimes investigators for Syria cautioned world powers preparing military action against Islamic State fighters that the 'laws of war' would apply to them and they must do everything possible to protect civilian populations.
They said ISIS, which controls swathes of Iraq and Syria, had committed massacres of religious and ethnic groups and abused women. The government of President Bashar al-Assad, however, remained responsible for the majority of civilian casualties.
World powers meeting in Paris on Monday gave public backing to military action to fight Islamic State fighters in Iraq. The US military struck an Islamic State target southwest of Baghdad, US Central Command said, in an expansion of the Obama administration's campaign against the group that has seized large swathes of Iraq and Syria.
"As military action on ISIS positions seems increasingly likely, we remind all parties that they must abide by the laws of war and most particularly, the principles of distinction and proportionality. Serious efforts must be made by all parties to preserve civilian life," Paulo Pinheiro, chairman of the UN commission of inquiry, told the Human Rights Council on Tuesday.
Air strikes against Islamic State fighters have already stirred complaints in Sunni areas of civilian casualties. Prime Minister Haider al-Abadi said on Saturday he had ordered his forces to refrain from any strikes in civilian areas, even towns occupied by IS militants.
The rules of war embodied in the Geneva Conventions require warring sides to distinguish between military and civil objects, such s schools and hospitals, and to carry out operations that are proportionate to the perceived threat.
Washington has been trying to build a coalition to fight Islamic State since last week when President Barack Obama pledged to destroy the militant group. France on Monday sent jets on a reconnaissance mission to Iraq, a step towards becoming the first ally to join the US-led air campaign there.
The countries that did attend the Paris talks made no mention at all of Syria, where US diplomats face a far tougher task building an alliance for action.
Islamic State forces in Syria have brutally and publicly executed civiilans, as well as captured rebel fighters and government soldiers over the past two months, Pinheiro said.
"Recent events in Iraq lay bare the threat ISIS poses for religious and ethnic minorities inside Syria," Pinheiro said.
The group killed hundreds of civilians at Al-Shaar gas fields in eastern Homs and there are reports of hundreds from the al-Sheitaat clan in Deir al-Zor being executed, he said.
"Women living in ISIS-controlled areas have been banned from public life...Some women have been stoned to death, ostensibly for adultery," Pinheiro told the Geneva forum.
Indoctrination of children is a priority for the insurgents and youth are made to take part in hostilities, he said.
The group is "the most recent beneficiary" of inaction by the UN Security Council and the prevailing impunity, according to the independent experts who include former UN war crimes prosecutor Carla del Ponte.