By John Davison and Suleiman Al-Khalidi BEIRUT (Reuters) - Hezbollah set off a bomb targeting Israeli forces at the Lebanese border on Monday in an apparent response to the killing in Syria last month of a prominent commander, triggering Israeli shelling of southern Lebanon. Israel has struck its Iran-backed Shi'ite enemy Hezbollah in Syria several times, killing a number of fighters and destroying weapons it believes were destined for the group, whose support for President Bashar al-Assad has been crucial in the country's civil war. Israel's army said Monday's blast, targeting military vehicles in the Shebaa farms area, prompted Israeli forces to respond with artillery fire.