U.N. war crimes investigators in Syria have warned world powers preparing military action against fighters from the Islamic State of Iraq and al-Sham (ISIS) that the laws of war would apply to them and they must do everything in their power to protect civilians. The investigators say that while ISIS, which controls large areas of Iraq and Syria, has massacred religious and ethnic groups as well as women and children, it is the government of President Bashar al-Assad, however, that has been responsible for most civilian casualties.World powers meeting in Paris on Monday pledged public support for military action against the ISIS fighters in Iraq. The American military hit an ISIS target in southern Baghdad, according to the U.S. Central Command. "As the military action on ISIS positions seems more likely, we remind all parties that they must respect the laws of war and in particular the principles of distinction and proportionality. Serious efforts must be made by all parties to protect civilian life," Paulo Pinheiro, chairman of the Commission of Inquiry of the U.N., said the Human Rights Council on Tuesday.Air strikes against ISIS combatants have already created complaints in Sunni areas of civilian casualties. Prime Minister Haider al-Abadi said Saturday that he had ordered his forces to refrain from strikes in civilian areas, even cities occupied by ISIS militants. The rules of war set out in the Geneva Convention require belligerents to distinguish between military and civilian targets, like schools and hospitals, and to conduct operations that are proportionate to the perceived threat.Washington has tried to build a coalition to fight ISIS over the last week after President Barack Obama promised to destroy the militant group. On Monday, France sent jets on a reconnaissance mission in Iraq, a closer step to the first allied air campaign.The countries that attended the talks in Paris made no mention of Syria, where American diplomats are facing a much more difficult task to build an action coalition. "Recent events in Iraq lay bare the threat ISIS pose for religious and ethnic minorities inside Syria," said Pinheiro.The group has killed hundreds of civilians in the Al-Shaar gas fields in eastern Homs and there are reports of hundreds of people from the Sheitaat clan fleeing in Deir al-Zor, he said. "Women living in areas controlled by ISIS were banned from public life... Some women have been stoned to death, allegedly for adultery," Pinheiro told the Geneva forum.The indoctrination of children is a priority for the insurgents and young people are made to take part in hostilities, he said. The group is "the most recent beneficiary" of inaction by the U.N. Security Council and the prevailing impunity, according to the independent experts who include former U.N. war crimes prosecutor Carla del Ponte. But the government of President Bashar al-Assad "remains responsible for the majority of civilian casualties, killing and wounding dozens of civilians every day - both from a distance using shelling and aerial bombardment closely with its checkpoints and its interrogation rooms, "said Pinheiro. "Over the past two months, Deraa was bombarded with little sign of the government to try to distinguish between civilians and members of armed groups," he said.Many areas under bombardment or siege are yielding to local truces, a "measure of the success of the government's 'starvation or submission strategy'," he said. "Checkpoints are often the starting point of a horrible trip disappearances, torture, sexual abuse, and for many, death," said Pinheiro.The U.N. investigators released 12 testimonies from among 3,200 collected in their interviews with Syrian refugees and defectors in neighbouring countries and by Skype within Syria. "They include a child injured in a missile attack on a school in Aleppo city, a man tortured in Damascus Mezzeh detention facility, and a pregnant woman left adrift after losing her husband and parents," Pinheiro said. "Thousands of men, women and children have been beaten, electrocuted and hung for hours by their wrists from walls." Women have given "shattering testimonies" of torture, rape and sexual assault by government forces, he said.