BEIRUT (Reuters) - Hezbollah’s top military commander Mustafa Badreddine has been killed in a blast at a base near Damascus airport, the Lebanese Shi’ite group said on Friday, one of the biggest blows to its leadership the Iranian-backed organization has ever sustained.

Hezbollah did not immediately say on Friday who it blamed for the attack, but its deputy leader Sheikh Naim Qassem said there were clear indications of who was behind it, and the group would announce the outcome of its investigation within hours. There was no immediate claim of responsibility.

At least one Hezbollah figure blamed the group’s age-old enemy Israel, which has struck Hezbollah targets in Syria several times in the past since civil war started there in 2011. Israel declined to comment, but a former Israeli official said his country would be glad Badreddine was dead.

Hezbollah also has many other foes in Syria, where it fights in support of the government of President Bashar al-Assad against a range of Sunni Muslim groups including Islamic State.

Thousands of Hezbollah fighters and leaders gathered at a mosque in Hezbollah’s stronghold in southern Beirut and gave Badreddine a military funeral, waving Hezbollah flags. They chanted Shi’ite religious slogans, as well as “Death to America” and “Death to Israel”.

Speaking at the funeral, Qassem also vowed that the group would continue on the “path” of Badreddine.

In a letter, Iran’s Foreign Minister Mohammad Javad Zarif extended condolences “for the martyrdom of this great jihadist ... who embodied devotion and vigor and was legendary in his defense of high Islamic goals and his defense of the Lebanese people who resist oppression and terrorism.”

The U.S. government believed Badreddine, 55, was in charge of Hezbollah’s military operations in Syria.

He is the most senior Hezbollah official killed since 2008 when his brother-in-law, long-serving military commander Imad Moughniyah, was blown up by a bomb planted in his car in Damascus that Hezbollah blamed on Israel.

The latest killing follows other recent losses for Hezbollah and Iran in Syria, despite Russian military intervention in support of Assad and his allies in a five year multi-sided civil war that has drawn in neighboring states and world powers.

At least four prominent figures in Hezbollah have been killed since January 2015. A number of high-ranking Iranian officers have also been killed, either fighting Syrian insurgents or in Israeli attacks.

Hezbollah said it was investigating whether the explosion at the base was caused by an air strike, a missile attack or artillery bombardment. It did not say when he was killed.

“This is an open war and we should not preempt the investigation but certainly Israel is behind this,” said Nawar al-Saheli, a Hezbollah member of Lebanon’s parliament, hinting at the prospect of retaliation: “The resistance will carry out its duties at the appropriate time.”

Israel never confirms or denies allegations of targeted killings of individuals abroad. When asked by an interviewer on Israel Radio about possible Israeli involvement, cabinet minister Zeev Elkin, a confidant of Prime Minister Benjamin Netanyahu, declined to comment.

Hezbollah is Lebanon’s most powerful political and military group, having grown stronger since forcing Israel to end its 22-year occupation of southern Lebanon in 2000. The sides fought a 34-day war in 2006, their last major conflict.

Israel deems Hezbollah its most potent enemy and worries that it is becoming entrenched on its Syrian front and acquiring more advanced weaponry.

“We don’t know if Israel is responsible for this,” Yaakov Amidror, a former national security adviser to Netanyahu, told Israel’s Army Radio. “Remember that those operating in Syria today have a lot of haters without Israel.”

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“But from Israel’s view, the more people with experience, like Badreddine, who disappear from the wanted list, the better,” he said.

A U.S. Department of the Treasury statement detailing sanctions against Badreddine last year said he was assessed to be responsible for the group’s military operations in Syria since 2011, and he had accompanied Hezbollah leader Sayyed Hassan Nasrallah during strategic coordination meetings with Assad in Damascus.

U.S. Army Colonel Steve Warren, a Baghdad-based spokesman for the U.S.-led coalition effort against Islamic State in Syria and Iraq, said it was too soon to assess what impact Badreddine’s death might have on Hezbollah but noted that it had suffered heavy casualties in Syria.

“But with regards to this specific strike, who took it and what the downstream impact is going to be of losing this leader – it’s simply too soon to tell,” he said.

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HIJACKERS SOUGHT HIS RELEASE

Announcing his death, Hezbollah quoted Badreddine as saying he would return from Syria victorious or as a martyr. A photo released by the group showed him before his death, smiling and wearing a camouflage baseball cap.

Badreddine’s death sparked wide condemnation from Lebanese political allies. “His martyrdom is a big loss for the Lebanese in their fight against Israeli-Zionist aggression and Takfiri terrorism,” Finance Minister Ali Hassan Khalil told Hezbollah’s al-Manar TV, in reference to Israel and Sunni militant groups.

“His loss will leave a vacuum but the lesson is to continue on the path that he chose -- resistance and Jihad until victory is achieved.”

Badreddine was sentenced to death in Kuwait for his role in bomb attacks there in 1983. He escaped from prison in Kuwait after Iraq, under the leadership of Saddam Hussein, invaded the country in 1990.

His release from jail in Kuwait was one of the demands made by the hijackers of a TWA flight in 1985, and of the hijackers of a Kuwait Airways flight in 1988.

For years, Badreddine masterminded military operations against Israel from Lebanon and overseas and managed to escape capture by Arab and Western governments.

Badreddine was also one of five Hezbollah members indicted by the U.N.-backed Special Tribunal for Lebanon in the 2005 killing of statesman Rafik al-Hariri, one of Lebanon’s most prominent Sunni Muslim figures. Hezbollah denied any involvement and said the charges were politically motivated.

Around 1,200 Hezbollah fighters are estimated to have been killed in the Syrian conflict. These include prominent figures Samir Qantar and Jihad Moughniyah, the son of Imad Moughniyah, who were killed in separate Israeli attacks last year.

Hezbollah responded in both cases, though the incidents were contained, with the sides seeking to avoid any repeat of the 2006 war which exacted a heavy price in Israel and Lebanon.