This is the only term I can think of, but note that it usually means that the material is specifically used to stop flames. I give this one medium confidence at best.

Problem is in English you can't use 'difficult' as an adverb. Maybe that's why in the below link they have just used the term 'difficult to ignite/burn'.

I think this pretty much hits the nail on the head... Nonflammable, fireproof, flame-proof, hard-to-burn, difficult-to-burn are also ok...