Sheikh Naim Qassem has been the acting head of Hezbollah since its longtime leader Sayyed Hassan Nasrallah was killed as part of an Israeli offensive that has taken out many of the group's senior officials.

Qassem made a defiant televised speech Tuesday, claiming that the group's military capabilities are intact and Israelis will only suffer further as fighting continues.

Like Nasrallah, Qassem is one of the founding members of the Shiite political party and armed group, but he is widely seen as lacking the former leader's charisma and oratory skills.

Still, the white-turbaned cleric with a gray beard has often been the public face of the group. After Nasrallah went underground out of fear of being assassinated by Israel, appearing only in televised speeches, Qassem continued to show up at rallies and ceremonies, and he has sat for interviews with foreign journalists.

Mohanad Hage Ali, a senior fellow at the Carnegie Middle East Center think tank who researches Hezbollah, said that Qassem is perceived by many as "more extreme" than Nasrallah, at least in his public statements.

In practice, however, his power within the group was limited under Nasrallah. Hashem Safieddine, a cousin of Nasrallah who oversees the group's political affairs — not Qassem — was generally regarded as the leader's heir apparent. But no announcement has been made, and Safieddine has not appeared publicly or made any public statements since Nasrallah's death and has been reportedly targeted in an Israeli strike on south Beirut.

Qassem was born in the town of Kfarfila in southern Lebanon and studied chemistry at the Lebanese University before working for several years as a chemistry teacher.

At the same time, he pursued religious studies and participated in founding the Lebanese Union for Muslim Students, an organization that aimed to promote religious adherence among students.

In the 1970s, Qassem joined the Movement of the Dispossessed, a political organization founded by Imam Moussa Sadr that pushed for greater representation for Lebanon's historically overlooked and impoverished Shiite community. The group morphed into the Amal movement, one of the main armed groups in Lebanon's civil war, and now a powerful political party.

He then joined the nascent Hezbollah, formed with support from Iran after Israel invaded Lebanon in 1982 and occupied the country's southern region.

From 1991, he served as deputy secretary-general of the group, initially under Nasrallah's predecessor, Abbas Mousawi, who was killed by an Israeli helicopter attack in 1992.