Celine Dion, Jennifer LopezMariah Carey and the Backstreet Boys are standing firm ... they will all take the stage in Las Vegas despite an ISIS threat to hit the Strip.

We contacted various people connected to all 4 acts, and are told they have not wavered in the wake of an ISIS recruitment video soliciting lone wolf attacks on the Las Vegas Strip.

Security will be ramped up in a big way -- both because of the Ariana Grande concert bombing and the new video -- but the shows will proceed. Celine and J Lo took the stage Wednesday night and will perform again this weekend. The Backstreet Boys are set to perform next month and Mariah shortly thereafter.

That said, if people have purchased tickets but now want refunds because they're afraid to attend, our sources at The Colosseum at Caesars -- that's where Celine and Mariah perform -- tell us people will "likely" get a refund.

As for Lopez and BSB ... Planet Hollywood wouldn't give us a clear answer on the subject of refunds.

OUR HEARTS WILL GO ON