Mustafa Badreddine, a senior commander in the Lebanese Hezbollah militia, was killed in an explosion this week in Syria, the Iran-allied group said Friday.
Hezbollah said the initial investigation showed that Badreddine, 55, died Tuesday night in a large explosion that targeted one of its bases near the Damascus airport.
"[The] investigation now is concentrating on whether the blast was caused by bombardment, a missile or an airstrike," Hezbollah said in a statement.
Lebanese media reported earlier that Badreddine had been killed in an Israeli air raid.
Neither the White House nor the US State Department could not confirm Badreddine's death.
"The thing that I can confirm is that there were no US or coalition aircraft in the area where he was reported to be killed," White House spokesman Josh Earnest said.
State Department spokesman John Kirby reiterated that Hezbollah is a US-designated foreign terrorist organization and described Badreddine as "a deeply committed terrorist."
In 2012, the US Treasury imposed sanctions on him and other Hezbollah leaders, citing the movement's "active support" to al-Assad's regime and its role in "terrorist activities."
Thousands of Hezbollah followers gathered late Friday in Beirut's southern suburbs for Badreddine's funeral.
Hezbollah deputy chief Naeem Qassem promised that the group will reveal in the coming hours who was behind Badreddine's killing.
"Israel and the Tikfri [militant] groups have received deadly strikes that were carried out by the martyr," Qassem said without elaborating.
Hezbollah followers wearing military fatigues lined the streets of Beirut's southern suburbs, a stronghold of the group, as black-clad women clad wept for the slain commander.
Badreddine's coffin, draped in the yellow Hezbollah flag, was carried through the streets of the area as mourners chanted: "May God bless your martyrdom."
Some mourners shouted "God bless Hezbollah" while raising their fists in the air.
Badreddine reportedly led Hezbollah's operations since 2011 in Syria, where the group has been fighting alongside the forces of President Bashar al-Assad against rebels.
Al-Assad, a Hezbollah ally, sent his condolences through Syrian Ambassador to Lebanon Ali Abdel-Kareem, Hezbollah television al-Manar reported.
In a letter of condolence to Hezbollah chief Hassan Nasrallah, Iranian Foreign Minister Mohammad Javad Zarif of Iran said the military commander's death would reignite the militia's fighting spirit against Israel and the Islamic State extremist movement in Syria.
"Mustafa Badreddine dedicated his whole life to the fight against injustice and terrorism," Zarif said in comments carried by Iranian state news agency ISNA.
Badreddine had lived in Syria in recent years.
"I will not return from Syria unless I am a martyr, carrying the banner of victory," al-Manar television quoted Badreddine as previously saying.
Badreddine and five other Hezbollah members were indicted by the UN-backed Special Tribunal for Lebanon over the 2005 assassination of former Lebanese prime minister Rafik Hariri.
Badreddine was linked to the 1983 bombing of the US Marine barracks in Beirut that killed 241 personnel, and was the convicted mastermind of deadly bombings targeting the US and French embassies in December 1983 in Kuwait City. He was sentenced to hang in Kuwait but eventually escaped after years in prison.
Badreddine is the third high-ranking Hezbollah commander slain in the last two years in Syria. Hezbollah is believed to have lost more than 1,200 fighters in the Syrian conflict, which arose after peaceful pro-democracy protests in March 2011.