In its 10th such report since the war began 4-1/2 years ago, the UN Human Rights Council urges the international community to help Syrian civilians.
"The global failure to protect Syrian refugees is now translating into a crisis in southern Europe," the report says, alluding to the increased flow in recent months.
Overall, it offers few new insights into a grinding civil war.
"Civilians are suffering the unimaginable as the world stands witness," said panel chair Paulo Sergio Pinheiro.
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It says warring factions have used tactics like encircling populated areas that have caused starvation, malnutrition and chronic illness among besieged residents.
The report highlights abuses by many combatants including President Bashar Assad's forces, the Islamic State group and al-Qaida-backed Nusra Front.
It says Islamic State fighters have adopted new tactics such as hit-and-run attacks and suicide car-bombings following battlefield losses to Kurdish fighters backed by US-led coalition air power.