I am reviewing a paper A for a Journal X. To be specific, the area is Mathematics. The works is built on a previous work B (already published) of the same author in another Journal Y (also a peer-review Journal but perhaps less prestigious). What I have seen after reading of A is that if the results of B are correct then the results of A are also correct. After looking at the paper B, it seems to me that the proofs are less convincing and I am uncertain of their correctness (I don't have counterexamples to show their incorrectness). If the results of B are correct, the paper A deserves to be published in the Journal X.
How can I deal with this situation?