BEIRUT –  The top commander of Lebanon's militant Hezbollah was killed in Syria by insurgent shelling, Hezbollah said Saturday, vowing to continue its involvement in Syria's civil war.

Mustafa Badreddine became the highest ranking casualty for Hezbollah since the group joined Syria's civil four years ago.

The group said the blast that killed Mustafa Badreddine near the Damascus International Airport was caused by artillery shelling by "takfiri" groups, a term Hezbollah uses to refer to Sunni extremists.

The area on the southern edge of the Syrian capital is known to host positions of several militant groups including al-Qaida's branch in Syria, known as the Nusra Front.

Rami Abdurrahman who heads the Britain-based Syrian Observatory for Human Rights said that according to his network of activists in the area on the ground, there has been no shelling in the airport area since Wednesday.

"Hezbollah must come forward with proof about the death of its commander," Abdurrahman said by telephone.

Hezbollah's statement said Badreddine's killing will only boost the group's "will and intention to continue fighting these criminal gangs until they are defeated."

It added that defeating insurgent groups in Syria was "the wish" of Badreddine who was also known among the group's ranks as Zulfiqar.

Hezbollah's statements hinted that the group will continue to be deeply involved in the conflict next door that has killed more than 250,000 people since 2011, including more than 1,000 Hezbollah fighters.

"It is the same battle against the American-Zionist project that the terrorists are spearheading," Hezbollah's statement said.

Hezbollah has sent thousands of fighters to Syria to back President Bashar Assad's government against militants trying to remove him from power.

The group announced Badreddine's death on Friday without saying when the attack occurred. It said at the time that an investigation has been launched into the cause of the blast.

The 55-year-old Badreddine had directed Hezbollah's operations in Syria since its fighters joined Assad's forces in 2012, the group's biggest-ever military intervention outside of Lebanon. Thousands of guerrillas fighting alongside Syria's military were crucial to tipping the battlefield in the government's favor on multiple fronts, from the suburbs of Damascus to the northern province of Aleppo.

With Badreddine's death, Hezbollah is likely to rely on a younger generation of commanders, moving away from the veterans who came of age during Lebanon's 1975-1990 civil war or during Hezbollah's 18-year war against Israel's occupation of southern Lebanon, which ended in 2000.

One possible successor, Ibrahim Aqil, is among the last major figures from that generation. A member of Hezbollah's highest military body, the Jihad Council, Aqil has been involved in the Syria fighting and is suspected in hostage-takings in the 1980s and a bombing campaign in Paris in 1986.