The head of the U.N. Commission of Inquiry on Syria is warning that an all-out conflict in the last major rebel stronghold in Idlib province "could generate an unimaginable human rights and humanitarian catastrophe."

Paulo Pinheiro says that would likely amplify human rights violations on "a massive scale."

Pinheiro told a news conference Thursday that the current aerial and ground offensive by the Syrian government and its allies is "a serious escalation" resulting in scores of civilian casualties and the displacement of over 150,000 people in just one week.

He expressed concern about tens of thousands of civilians displaced during the final campaign to rout Islamic State extremists from territory it held in northern Syria. He says many are "being held in limbo under dire humanitarian conditions" and "being treated as security threats."