The leader of Lebanon's Hezbollah group has strongly denounced the Islamic State group's beheading of a group of Egyptian Christian hostages in Libya.

In a speech to hundreds of his supporters in southern Beirut, Sheikh Hassan Nasrallah says the CIA and Israel's Mossad are behind the extremist group, which serves their interests.

He claims Israel is the only country which has not been affected and does not consider the group a threat.

The Iranian-backed Shiite Hezbollah group, which has sent thousands of its fighters to Syria to help shore up President Bashar Assad's forces, has positioned itself as a vanguard against Sunni extremists such as the Islamic State group.

Nasrallah called on regional governments to work together to confront IS, whose crimes "are a threat to the entire universe."