UN forces will not intervene although there may be some humanitarian efforts if such is practical. If so, they will probably offer it to both sides, but Russia won't allow any such thing on their soil (or their puppets') while the Ukrainians won't likely be in a position to deny aid.
There will be indirect NATO/US/European involvement but not direct involvement. (i.e. no such forces actually fighting Russian forces, but support via arms shipments, intelligence sharing, etc., while putting increased economic pressure on Russia via sanctions and embargoes, and possibly cyberattacks if they think they can do so with plausible deniability.)