An undated handout photo released on May 13, 2016, by Hezbollah's media office shows Mustafa Badreddine at an undisclosed location. (Agence France-Presse via Getty Images)

A Hezbollah commander described as the leader of its militia forces in Syria was killed in a mysterious blast in Damascus, the group said Friday. The explosion targeted a figure known for both a playboy lifestyle and links to major terror attacks dating back to the bombing of U.S. Marine barracks in Beirut in 1983.

The death of Mustafa Badreddine represents a significant blow to the Iran-backed group as it reaches beyond its strongholds in Lebanon to aid President Bashar al-Assad against rebel factions, including some groups backed by the United States and its allies.

It also highlighted the depth of Hezbollah involvement in Syria, with some of its most senior leaders apparently working closely with Assad and his generals. And it is unlikely that the loss of Badreddine will cause Hezbollah to alter its commitment to Assad, whose country provides critical support and supply routes.

There were no clear clues on the cause of the explosion, but a television station allied with Hezbollah pointed the finger at the group’s longtime foe, Israel.

Badreddine, 55, is the highest-ranking Hezbollah official killed in the Syria campaign, and the most senior since the 2008 killing in Damascus of his mentor, Imad Mughniyah. That attack was believed to have been carried out by U.S. and Israeli agents, but neither side has publicly acknowledged any role.

Hezbollah supporters carry the coffin of slain commander Mustafa Badreddine in Beirut on May 13. (Hassan Ammar/Associated Press)

Over more than three decades, Badreddine’s movements and activities were shrouded in secrecy and speculation. But what leaked out helped build a reputation for both bon vivant-like excess and headline-grabbing bloodshed.

He was linked to deadly attacks in 1983 on U.S. and French embassies in Kuwait and was suspected of playing a role in the Beirut barracks bombings that killed 241 U.S. servicemen and 58 French ones.

He also was among four people indicted by a U.N. tribunal for the 2005 assassination of Lebanese Prime Minister Rafiq al-Hariri.

In U.N. transcripts, the prosecution described Badreddine as using at least two aliases and cover stories, including as a Beirut jeweler named Sammi Issa. At points in his life, according to the transcripts, he owned an apartment in an upscale area near the Lebanese capital and had several “concurrent” girlfriends, as well as a boat and an “expensive Mercedes” registered in other names.

A Hezbollah statement said a “huge explosion” killed Badreddine but did not say where it happened or speculate on responsibility. It promised to release details of an internal investigation later.

But Beirut-based Al-Mayadeen TV, which is close to Hezbollah, blamed the killing on Israel, which has carried out a number of air raids against the group in Syria in recent years. In 2006, Israel fought a brief war with Hezbollah but failed to dislodge the group from strongholds in southern Lebanon.

In Hezbollah-dominated south Beirut, thousands of mourners turned out for Badreddine’s funeral late Friday, waving the group’s yellow flags and chanting slogans.

Apparent Israeli attacks in Syria have killed top Hezbollah militants and destroyed what analysts say were high-powered weapons — including missiles provided by Iran — that the group could have used against Israel.

Israel generally neither confirms nor denies involvement in such attacks.

According to the U.S. government, Badreddine commanded Hezbollah’s substantial military operations in Syria — an intervention involving thousands of the Shiite group’s militants who have been waging intense ground battles against the Sunni-led rebellion.

The group’s well-trained fighters have played a crucial role in defending the Syrian government, whose forces are also aided by thousands of Shiite fighters from Iran, Iraq and Afghanistan.

But the fighting has taken a heavy toll on Hezbollah, resulting in well over 1,000 of its militants being killed.

Relatively little is known, however, about Badreddine, a Shiite Muslim born in 1961. For decades, analysts say, he operated under a cloud of extreme secrecy as he plotted attacks against Israeli, U.S. and Lebanese targets.

The clandestine fog is rooted in general in fear of Israel, which runs spies and moles to foil Hezbollah operations and assassinate its leaders.

In 1992, Israeli helicopters killed Hezbollah’s leader, Abbas Musawi, his wife and young son. Late last year, an Israeli airstrike in Damascus killed Samir Kuntar, a prominent Hezbollah militant who had spent three decades in an Israeli prison.

“They are secretive and shady because if they aren’t, they get killed more quickly,” said Nicholas Blanford, a Beirut-based journalist and author of “Warriors of God: Inside Hezbollah’s Thirty-Year Struggle Against Israel.”

According to some accounts, Badreddine also had a reputation for extravagance with his assumed identities.

“He had several concurrent girlfriends and was seen regularly in restaurants and cafes socializing with his friends,” the U.N. prosecution wrote in the documents from January 2014. “He was also accompanied by armed body-guards.”

The prosecution accuses Badreddine of playing a key role in Hariri’s assassination, an event that has continued to be a major source of instability in Lebanon.

Matthew Levitt, a Hezbollah expert at the Washington Institute for Near East Policy, described Badreddine as having the ear of Hezbollah’s leader, Hasan Nasrallah, and joining him in meetings in Damascus with Assad on a regular basis to discuss the Syrian conflict.

Levitt partly attributed Badreddine’s influence in the group to his relationship with Mughniyah, who headed Hezbollah’s militant operations.

The two were cousins and brothers-in-law, and they were suspected of helping plot the barracks bombings after U.S. forces aiding a peacekeeping mission during Lebanon’s civil war.

“They planned the bombing of the Marine barracks and reportedly sat on top of a building together with binoculars to watch the attacks,” Levitt said.

One of the last public appearances that Badreddine made was in Beirut for the funeral of Mughniyah’s son, Jihad, who was killed by a suspected Israeli drone strike in southern Syria last year.

After the 1983 bombings of the U.S. and French embassies in Kuwait, authorities there arrested him and sentenced him to death. He managed to escape imprisonment when Iraq under Saddam Hussein invaded Kuwait in 1990.

Suzan Haidamous in Beirut, Brian Murphy in Washington and Ruth Eglash in Jerusalem contributed to this report.