Top Hezbollah commander Mustafa Badreddine has been killed in an attack in Syria, the Lebanese group said on Friday, the biggest blow to the Iranian-backed organization since its military chief was killed in 2008.

Badreddine, 55, was one of the highest ranking officials in the group and was believed by the U.S. government to be responsible for Hezbollah's military operations in Syria, where it is fighting alongside the Syrian President Bashar al-Assad.

The killing of Badreddine, a brother-in-law of late Hezbollah military commander Imad Moughniyah, is the latest big loss sustained by Hezbollah and Iran in Syria despite Russian military intervention in support of Assad and his allies.

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 Badreddine had been killed in a big explosion targeting one of its bases near Damascus airport and an investigation was underway into whether it was caused by an airstrike, a missile attack, or artillery bombardment. It did not say when he was killed.

'Open war'

Nawar al-Saheli, a Hezbollah member of Lebanon's parliament, said Israel was behind the killing.

"This is an open war and we should not preempt the investigation but certainly Israel is behind this," he told the Hezbollah-controlled al-Manar TV station. "The resistance will carry out its duties at the appropriate time."

The Lebanese TV station al-Mayadeen reported Badreddine had been killed in an attack by Israel, which has struck Hezbollah targets in Syria several times since the conflict began in 2011.

There was no immediate response from Israel, which deems Hezbollah its most potent enemy and worries that it is becoming entrenched on its Syrian front and is acquiring more advanced weaponry.

Hezbollah, a political and military movement which is Lebanon's most powerful group, has 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.

Mustafa Badreddine is seen in this handout picture released by Hezbollah. (Provided/Hezbollah via Reuters)

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.

Yaakov Amidror, a former national security adviser to Netanyahu, said Badreddine's killing was good news for Israel but stopped short of saying Israel was responsible.

"We don't know if Israel is responsible for this," he told Israel's Army Radio. "Remember that those operating in Syria today have a lot of haters without Israel."

"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.

Announcing his death, Hezbollah cited Badreddine 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.

Hezbollah's al-Manar TV said he would be buried at 5:30 p.m. local time in the southern suburbs of Beirut.

Hijackers sought his release

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 UN-backed Special Tribunal for Lebanon in the 2005 killing of statesman Rafik al-Hariri. The group 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 fighters 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.

Hezbollah accuses Israel of carrying out the 2008 killing of Moughniyah, who was killed by a bomb in Damascus.