BEIRUT, Lebanon — Mustafa Amine Badreddine, Hezbollah’s top military commander, who was directing its operations in Syria and was accused in a string of deadly attacks stretching across decades, has been killed in Damascus, Hezbollah officials confirmed on Friday.

 

Mr. Badreddine, 55, had been overseeing Hezbollah’s forces in Syria, which have been decisive in keeping President Bashar al-Assad in power through five years of war with various rebel and militant Islamist groups seeking to topple him.

 

Mr. Assad is a close ally of Hezbollah’s patron, Iran, which has long provided a conduit to supply Hezbollah with weapons to battle Israel. Now, Hezbollah is the most powerful of several Iran-backed Shiite paramilitary groups that, along with Iranian forces, are playing an ever more prominent role on the battlefield in Syria.

 

It remained unclear who was behind Mr. Badreddine’s death: Israel, or one of the insurgent groups Hezbollah has been fighting.

 

Hezbollah, which confirmed the death on Al Manar, its television network, said that Mr. Badreddine had died in a “huge blast” near the Damascus airport, in which several of the group’s fighters were wounded. “The investigation will find out the nature of the blast as well as its reasons, and whether it was a result of an airstrike or rocket attack,” it said.

 

The timing of the attack was not provided. A Beirut-based television network, Al Mayadeen, which is also close to Hezbollah, initially reported that Mr. Badreddine had been killed in an Israeli airstrike, but it later removed that report.

 

The Israeli prime minister’s office declined to comment on Mr. Badreddine’s death, as did the Israel Defense Forces and the country’s Foreign Ministry.

 

Mr. Badreddine was one of four people being tried in absentia for the 2005 assassination of a former prime minister of Lebanon, Rafik Hariri, in a suicide bombing that killed him and 22 others. The trial is being held by a special United Nations-backed tribunal near The Hague. A billionaire businessman, Mr. Hariri was Lebanon’s most prominent politician after a 15-year civil war ended in 1990 with a power-sharing agreement.

 

Hezbollah has denied involvement in Mr. Hariri’s assassination and said that the charges were politically motivated. The indictments of Mr. Badreddine and other Hezbollah members were handed down in 2011 by the tribunal, which acknowledged that it had found no smoking gun but was instead relying largely on circumstantial evidence.

 

The killing of Mr. Badreddine was the biggest blow to Hezbollah since the death in 2008 — in a bomb attack in Damascus — of his brother-in-law Imad Mugniyah, who was behind the 1983 bombing of the American Marine barracks in Beirut that killed 241 people, and other acts of violence.

 

After that, Mr. Badreddine, known among the group’s ranks as Zulfiqar, became Hezbollah’s top military commander and an adviser to the group’s leader, Hassan Nasrallah.

 

“This martyrdom will be an incentive, same as those of late commanders, for more jihad, sacrifices and continuity,” a Hezbollah official, Hussein Haj Hassan, told Al Manar. “As he led the war against takfiris, the battles against takfiris will continue until victory is achieved.”

 

Takfiris are extremist Islamists who brand other Muslims, often those who disagree with their ideology, apostates. Some extremist Sunni groups operating in Syria, like the Islamic State and the Nusra Front, apply the apostate label to Shiites, who make up Hezbollah’s base and are a majority of the population of Iran, a Shiite theocracy.

 

A shadowy figure with an expertise in explosives, Mr. Badreddine used numerous aliases, including Elias Saab and Sami Issa. Until his death, the only widely circulated image of him was decades old, a black-and-white photograph of him as a much younger man wearing a suit. Hezbollah, in confirming the death, released a new image of him, in military attire.

 

Hezbollah said it would be receiving condolences starting Friday morning at its stronghold south of Beirut. A funeral was to be held Friday afternoon at a Shiite cemetery where Mr. Badreddine was to be buried next to Mr. Mugniyah.

 

Mr. Badreddine was suspected of involvement in the deadly 1983 bombings of the American and French Embassies in Kuwait. He was detained and sentenced to death in Kuwait, and he was imprisoned for years until he fled prison in 1990, when Iraq, under President Saddam Hussein, invaded Kuwait.

 

Ron Ben-Yishai, an Israeli military analyst, wrote on the news site Ynet that Mr. Badreddine had had many enemies. Israel, for example, suspected that he had played a role in the 1992 attack on the Israeli Embassy in Buenos Aires and a 2012 attack on a tourist bus in Bulgaria, in which five Israelis were killed.

 

But he doubted Israel was responsible for the killing. “Israel as a state does not settle accounts, but rather thwarts future threats,” Mr. Ben-Yishai wrote, adding that Mr. Badreddine “was more of a threat to the rebels in Syria and Lebanon than he was to Israel.”

 

Mr. Ben-Yishai wrote, “That is why it can be assumed with much certainty that the background to his assassination was internal Lebanese conflicts.”

 

Over the past 30 years, Israel has killed some of Hezbollah’s top leaders. In 1992, Israeli helicopter gunships ambushed the motorcade of Mr. Nasrallah’s predecessor, Sheik Abbas Musawi, killing him, his wife and son, and four bodyguards. Eight years earlier, another Hezbollah leader, Sheik Ragheb Harb, was gunned down in southern Lebanon.

 

Hezbollah has taken a hit to its reputation for its intervention in civil wars in Yemen, where it backs Shiite Houthi rebels who are enemies of Saudi Arabia, and in Syria. It was once lauded in Lebanon and the Arab world, by many Sunnis and Shiites alike, as a heroic movement that resisted Israeli aggression. But now, Hezbollah’s role in Syria has divided Lebanon and the Arab world — roughly, though not entirely, along sectarian lines. Those who see Mr. Assad as a brutal dictator view Hezbollah as helping him to crush a five-year-old insurgency led by the country’s Sunni majority.

 

The Arab League designated Hezbollah a terrorist organization in March, and shortly after, Saudi Arabia cut $4 billion in aid to Lebanese security forces after the Lebanese government declined to join resolutions critical of Iran and Hezbollah. Saudi Arabia and other predominantly Sunni Persian Gulf states have also cut Lebanese satellite broadcasts, expelled Lebanese expatriates they say have ties to Hezbollah and warned their citizens against traveling to Lebanon.