The UN Commission of Inquiry says Syria this year has witnessed levels of internal displacement not seen before in the seven-year conflict.
The agency said in a report released Wednesday that over 1 million Syrian men, women and children have been displaced so far this year "with most now living in dire conditions."
Syrian government forces backed by Russia and Iran captured wide areas around the country from rebels earlier this year.
The areas include southern regions on the fronts with Jordan and the Israeli-occupied Golan Heights as well as eastern suburbs of Damascus known as eastern Ghouta.
The agency also warns that a possible offensive on the northwestern province of Idlib "would generate a catastrophic" humanitarian crisis.
Regime forces have been massing troops on the edge of Idlib in preparation for an offensive on the last major rebel stronghold in Syria.
Meanwhile, United Nations Secretary-General Antonio Guterres on Tuesday appealed to Russia, Iran and Turkey to “spare no effort to find solutions that protect civilians” in Syria’s Idlib and said it was “absolutely essential” a full-scale battle was avoided.
“This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict,” he told reporters.
An estimated 3 million people live in Idlib - the last major stronghold of active opposition to Bashar al-Assad. The UN has described it as a “dumping ground” for people evacuated and displaced from elsewhere in Syria during the seven-year war.
Assad has vowed to retake the region, backed by his Russian and Iranian allies.
