Israel has stated that they're the victim of an act of war, but stopped short of saying they're at war with Lebanon. There are a few threads on officials public statements, but the letter to the UN below is probably the best to read. And yes, at this point in time Lebanon is clearly the belligerent party, not Syria or Iran, nations who are aiding them. Not in my opinion, but in the context of who Israel is at war with.
Israeli UN Ambassador's Letter to UN Secy.-Gen. Annan