How to review a paper that relies heavily on references to unpublished work? I am currently doing a (blind) review of a paper for a Journal in the engineering field. The manuscript seems okay so far but relies to a large extent on a unpublished paper, cited as Miller et al. forthcoming. That publication was not submitted along with the manuscript. The author seems to be involved in that publication, but is not co-authoring it. Since no journal or publisher is given, I am assuming that the manuscript is under preparation and to be submitted to a Journal. 

Furthermore, there is no summary of the central data and results e. g. in the supporting information. That makes it hard to follow some of the assumptions and conclusions. 


How should I proceed? 
And does this in principle justify a rejection of the paper?
Every matter in a paper should be verifiable, including the references. All results should be supported by relevant explanation with sufficient detail. 

Instead of rejecting the paper straight away, you may initially tell the author of correspondence (through the editor) to provide the unpublished cited materials. The author is to be obliged to provide you the tangible material from which you may assess the credibility of the facts cited. 

Bear in mind that all communication between the author and reviewer must go via the editor of the journal.