Lebanon's militant Hezbollah group said Friday that its top military commander Mustafa Badreddine was killed in an explosion in the Syrian capital of Damascus, a major blow to the Shiite group which has played a significant role in the conflict next door.

Badreddine, 55, had been supervising the group's involvement in Syria's civil war since Hezbollah fighters joined the battles along with Syrian President Bashar Assad's forces against militant groups trying to remove him from power, according to pro-Hezbollah media. Hezbollah, along with Iran, has been one of Assad's strongest backers.

Hezbollah said several others were wounded in the blast. It said it was investigating the nature of the explosion and whether it was the result of an air raid, missile attack or artillery shelling.

The Beirut-based Al-Mayadeen TV that is close to the group earlier said Badreddine was killed in an Israeli airstrike but later removed the report.

Badreddine ("Ba-dre-deen") was one of four people being tried in absentia for the murder of former Lebanese Prime Minister Rafik Hariri. The 2005 suicide bombing that killed Hariri and 22 others was one of the Middle East's most dramatic political assassinations. The trial is ongoing near The Hague, Netherlands. A billionaire businessman, Hariri was Lebanon's most prominent politician after the 15-year civil war ended in 1990.

Badreddine's death is the biggest blow to the militant group since the 2008 assassination of his predecessor, Imad Mughniyeh, who was killed in a bomb attack in Damascus. After that, Badreddine, known among the group's ranks as Zulfiqar, became Hezbollah's top military commander.

"Early information from the investigation shows that a strong explosion targeted one of our centers near the Damascus International Airport leading to the martyrdom of brother commander Mustafa Badreddine and wounded several others," Hezbollah said in a statement issued Friday.

Hezbollah said Badreddine was a "great jihadi leader" that he had joined "the convoy of martyrs on top of them his comrade and close friend Mughniyeh.

The group said it will be receiving condolences starting Friday morning in their stronghold south of Beirut.

Badreddine was the brother-in-law of Mughniyeh and was suspected of involvement in the 1983 bombings of the U.S. and French embassies in Kuwait that killed five people. He was detained in Kuwait and imprisoned for years until he fled jail in 1990 after Iraq's Saddam Hussein's forces invaded Kuwait.

Over the past 30 years, Israel has killed some of the group's top leaders. In 1992, Israeli helicopter gunships ambushed the motorcade of Sayyed Abbas Musawi, killing him, his wife, 5-year-old son and four bodyguards. Eight years earlier Hezbollah leader Sheik Ragheb Harb was gunned down in south Lebanon.

There was no immediate comment from Israel.

Hezbollah has paid a very steep price for its public and bloody foray into Syria's civil war. Once lauded in Lebanon and the Arab world as a heroic resistance movement that stood up to Israel, it has seen its popularity plummet, even among its Lebanese base, because of its staunch support for Assad.

The Arab League designated Hezbollah a terrorist organization in March. A month earlier, Saudi Arabia cut $4 billion in aid to Lebanese security forces after Lebanon's Foreign Minister Gibran Bassil declined to join Arab and Islamic league resolutions critical of Iran and Hezbollah.

The predominantly Sunni Gulf Arab states, led by the kingdom, have taken other punitive measures. They have warned their citizens against traveling to Lebanon as well as cut Lebanese satellite broadcasts, and closed a Saudi-backed broadcaster in Lebanon. The Gulf countries are also expelling Lebanese expatriates they say have ties to Hezbollah.

Hezbollah, which maintains a dominant militia force in Lebanon, has also aligned itself with the Saudi-opposed Shiite Houthi rebels in Yemen's civil war.