Nasrallah's long-serving deputy Naim Qassem is appointed new Hezbollah leader

Qassem is known as the public face of the militant group, rather than as a military tactician

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Hezbollah has announced Naim Qassem as its new secretary general, filling a crucial vacancy as the Iran-backed militant group comes under increasing Israeli military pressure that has wiped out a large proportion of its top echelons.

His predecessor Hassan Nasrallah was killed in an air attack on Beirut in September. Mr Qassem was Nasrallah's deputy for three decades, but he was not considered a likely successor until Israel started dismantling Hezbollah's command structure, reducing the pool of veteran figures who could take over.

Mr Qassem is an unknown quantity as far as the military and political capabilities that Hezbollah needs to survive the Israeli onslaught. He is widely seen as more of a theoretician than a tactician. In his deputy role he focused on public relations – mainly media interviews – and Hezbollah's electoral campaigns.

He was a disciple of Imam Musa Al Sadr, an eminent Lebanese Shiite cleric who disappeared in Libya in 1978, just after meeting dictator Muammar Qaddafi. Mr Al Sadr preached coexistence and even famously lectured at a church in downtown Beirut.

Shortly after Mr Al Sadr's disappearance, Mr Qassem left Amal, the Shiite movement that the imam had founded. His career took a more militant trajectory, as he became one of the first cadres of Hezbollah when it was founded by Iran's Islamic Revolutionary Guard Corps and Syrian intelligence in the 1980s.

In one interview, he said that training camps set up by the IRGC in the Bekaa Valley during Lebanon's civil war made people discover “a classy model of education, preparedness, morals and faith”.

After an inconclusive war with Israel in 2006, Mr Qassem's pronouncements grew more dismissive of the country, hinting that Israel might be too divided to win a new war against the group. Since the killing of Mr Nasrallah, he has said repeatedly that Hezbollah remains strong, despite Israel's invasion of south Lebanon and expanding aerial attacks across the country.

But Mr Qassem said that Hezbollah would sign a ceasefire with Israel, abandoning a previous stance that tied a ceasefire in Lebanon with a ceasefire in Gaza.

He was not the first choice to succeed Mr Nasrallah. Hashem Safieddine, Nasrallah’s maternal cousin and head of Hezbollah's executive council, had been seen as the likely new leader. However, Mr Safieddine was assassinated days after the killing of Nasrallah in another Israeli bombing of Beirut. Hezbollah only confirmed Mr Safieddine's death last week.

Mr Qassem was appointed deputy leader in 1991 by Hezbollah's then-secretary general Abbas Al Musawi, who was killed by an Israeli helicopter attack the following year.

The Israeli army has only made limited progress in its invasion of south Lebanon under intense fire from Hezbollah units. The militant group has ramped up the scope of its attacks on Israel, hitting military bases near Tel Aviv.