This is puffery from Iran. Does anyone believe Belarus is in possession of the state of the art S-300 that could threaten the IDF? Greece also has a version of the S-300 and allowed Israel to practice defeating it. It's doubtful Belarus could provide Iran anything more than an obsolete missile and propaganda.
High school teacher's union threatens strike (Haaretz)
from the article: Iran official: We have obtained the S-300 missile system