Hassan Nasrallah, the leader of the Islamist group Hezbollah, has praised the Hamas attacks on Israel in his first speech about the war.
Over the past month, Hezbollah has frequently targeted Israeli armour and military positions from across the border in Lebanon.
What is Hezbollah, and where is Lebanon?
Hezbollah is a Shia Muslim organisation which is politically influential and in control of the most powerful armed force in Lebanon.
It was established in the early 1980s by the region's most dominant Shia power, Iran, to oppose Israel. At the time, Israel's forces had occupied southern Lebanon, during the country's civil war.
Hezbollah has participated in national elections since 1992 and has become a major political presence.
Its armed wing has carried out deadly attacks on Israeli and US forces in Lebanon. When Israel withdrew from Lebanon in 2000, Hezbollah took credit for pushing them out.