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."
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The areas include southern regions on the fronts with Jordan and the 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.
Government forces have been massing troops on the edge of Idlib in preparation for an offensive on the last major rebel stronghold in Syria.