His threat is as powerful as North Korea's threat to wipe US cities out.If and when Israel decides to attack Iran, it would be so much warranted, there will be huge open and silent (think who this will be) support behind it. Iran would not know what hit them, let alone razing Israel's cities. Between the two, Hezbolllah would have a better chance at a greated damage to Israel than Iran. But I am sure Israel would think of that two and have a plan in advance..
from the article: Iran's Khamenei: We will destroy Israeli cities if attacked