In science, if you were to be about to try to support the claim by experimental or observational evidence, you might call it an hypothesis.

In philosophy, if you were about to support the claim by way of reason or argumentation, it could be called a proposition.

Ontologically, a fact is a piece of data, distinct from the person expressing it. What you are referring to is the expression of something erroneously purporting to be a fact.

If you knew it was wrong, it would simply be a lie.

If you thought it was right, it might be an hypothosis in the sense of something taken to be true for the purpose of argument or investigation; an assumption.

It is not a fact, until shown to be true.

I think in this case, assertion is most appropriate.