BEIRUT — The top military commander of Lebanon's Hezbollah was killed in an explosion near the Syrian capital of Damascus, the Shiite guerrilla group said Friday, the highest-level casualty yet in its intervention in the raging civil war next door.
The death of Mustafa Badreddine strikes a heavy blow to the militant group and underscores how its deployment in Syria backing President Bashar Assad has widened its circle of enemies beyond traditional foe Israel to include Sunni extremists and conservative Gulf monarchies.
The 55-year-old Badreddine had directed Hezbollah's operations in Syria since its fighters joined Assad's forces in 2012, the group's biggest ever military intervention outside of Lebanon. Thousands of guerrillas fighting alongside Syria's military were crucial to tipping the battlefield in the government's favor on multiple fronts, from the suburbs of Damascus to the northern province of Aleppo.
But it has come at a heavy price, with more than 1,000 Hezbollah fighters killed. Sounding a tone of defiance Friday, the group's deputy leader said they were undeterred.
"By killing you, they gave a new push to our drive that produces one martyr after another, as well as one commander after another," Naim Kassem said as the slain military chief was buried in a cemetery in southern Beirut.
Still, the slaying was unlikely to lead Hezbollah to reduce its role in Syria.
Hezbollah leader Sheikh Hassan Nasrallah has touted the war as necessary to protect Shiites — including in Lebanon — from Sunni extremists who have been at the forefront of the opposition, and Hezbollah's patron Iran has shown itself determined to keep its ally, Assad, in power.
"I do think it will affect their morale. This is not just their commander in Syria. This is one of the most elite and uniquely pedigreed Hezbollah personalities," said Matthew Levitt, director of the Stein Counterterrorism Program at the Washington Institute. But "I don't think they are going to waiver in their commitment" to fighting the Sunni extremists arrayed against Assad.
Those behind the explosion that killed Badreddine remained a mystery. The blast occurred near the Damascus airport, which is close to the Shiite shrine of Sayyida Zeinab, where Hezbollah has a strong presence and several military positions. The area was once a heavy battle zone, and although Hezbollah's fighters have largely succeeded in pushing the insurgents out, bombings have occurred there as recently as last month.
Hezbollah's traditional enemy, Israel, has assassinated its leaders in the past. But Sunni opposition forces, including militants like the Islamic State group or al-Qaida branch in Syria, the Nusra Front, could also be behind the blast.
Hezbollah said it was investigating whether the blast, which also wounded several others, was from an air raid, missile attack, artillery shelling or other cause. Kassem later hinted the group knew who was behind the killing, saying investigation results could be released Saturday.
The Beirut-based Al-Mayadeen TV, which is close to Hezbollah, initially said Badreddine was killed in an Israeli airstrike but later took down the report. Israeli officials refused to comment.
With Badreddine's death, Hezbollah is likely to rely on a younger generation of commanders, moving away from the veterans who came of age during Lebanon's 1975-1990 civil war or during Hezbollah's 18-year war against Israel's occupation of southern Lebanon, which ended in 2000.
One possible successor, Ibrahim Aqil, is among the last major figures from that generation. A member of Hezbollah's highest military body, the Jihad Council, Aqil has been involved in the Syria fighting and is suspected in hostage-takings in the 1980s and a bombing campaign in Paris in 1986.
"By default, we are witnessing the evolution of Hezbollah from a military unit dominated by soldiers who took part in the armed struggle against Israel to one that increasingly relies on younger recruits who weren't even alive during the Israeli occupation," said Bilal Saab, a resident senior fellow for Middle East security at the Brent Scowcroft Center on International Security, Atlantic Council.
Badreddine, like many of Hezbollah's top military operatives, was a shadowy figure. The only public image of him was a decades-old black-and-white photograph of a smiling young man wearing a suit until Hezbollah released a new image of him in military uniform Friday.
Accused of being the mastermind behind the assassination of former Lebanese Prime Minister Rafik Hariri in 2005, he was one of five people on trial in absentia at an international court in the Netherlands over the killing. Hezbollah has denied any role in Hariri's death.
Badreddine was a student of Imad Mughniyeh, Hezbollah's previous military chief who was considered one of the world's most-wanted terrorists by Israel and the United States. Mughniyeh, who is Badreddine's brother-in-law, was killed in a 2008 car bombing in Damascus that Hezbollah blamed on Israel.
An explosives expert, Badreddine was accused in the 1983 bombings of the U.S. and French embassies in Kuwait that killed five people. Sentenced to death in Kuwait in the 1980s, he fled in a dramatic escape after Saddam Hussein's forces invaded the oil-rich Gulf country in 1990.
His nom de guerre, Zulfiqar, referred to the double-headed sword of Imam Ali, the Prophet Muhammad's son-in-law and the Shiite sect's most sacred martyr, symbolizing a skilled fighter who killed many infidels during the early days of Islam.
The U.S. Treasury Department imposed sanctions twice on Badreddine for his involvement in the Syrian war, in 2011 and in 2015.
"This is a prince of Hezbollah, second only to the late Mughniyeh," said Levitt. Still, he said, Badreddine was a hot-headed, impetuous commander, making him unpopular in some quarters of the group, in contrast to Mughaniyeh, a "cold, calculating experienced operator."
Hezbollah has paid a steep price for its bloody foray into Syria's civil war, beyond its casualties. Once lauded in the Arab world as a heroic resistance movement that stood up to Israel, its staunch support for Assad has been criticized at home, even among its Lebanese support base.
Perhaps in response, Nasrallah and other Hezbollah officials have increasingly depicted Israel and its opponents in Syria — whether jihadi extremists or Saudi Arabia, the Sunni powerhouse that backs the Syrian opposition — as one and the same.
At Badreddine's funeral, chants by the thousands of mourners marching behind the coffin in the group's stronghold of southern Beirut reflected the expanding circle of the guerrilla group's foes. "Death to Israel!" ''Death to America!" and "Death to Al Saud!" they shouted — referring to the Saudi royal family.
Hezbollah fighters carried the coffin, draped with the group's yellow flag, as women on balconies tossed flowers and rice. It was nearly toppled at times by the jostling crowd as it was taken for burial near Badredinne's mentor, Mughniyeh.
"We will continue to confront Israel and we will continue to confront the Takfiris," Kassem declared, using a term for Sunni extremists.
"For us, there is only one enemy, which is Israel and those siding with it."