A loud blast was heard and a plume of smoke could be seen rising above Beirut’s southern suburbs - a stronghold of the Iran-backed Hezbollah - at around 7:40 p.m. (1640 GMT), a Reuters witness said.

Israeli Defense Minister Yoav Gallant said the strike killed Fuad Shukr, who "has the blood of many Israelis on his hands. Tonight, we have shown that the blood of our people has a price, and that there is no place out of reach for our forces to this end."

There was no immediate response from Hezbollah. The group has denied involvement in a rocket strike on the Israeli-occupied Golan Heights on Saturday that killed 12 youth in a football field in the Druze village of Majdal Shams.

A senior security source from another country in the region confirmed Shukr had died of wounds sustained in the strike.

Israel’s military said Shukr was the most important aide to Hezbollah Secretary-General Hassan Nasrallah, his adviser for wartime operations and in charge of Saturday’s attack.

Also read: Don’t bomb Beirut: U.S. leads push to rein in Israel’s response

On the other hand, Hamas leader Ismail Haniyeh was assassinated in the early hours of the morning in Iran, the Palestinian militant group Hamas said on Wednesday, describing the strike as a "severe escalation" that would not achieve its goals.

Iran’s Revolutionary Guards confirmed the death of Haniyeh, hours after he attended a swearing in ceremony for the country s new president, and said it was investigating.

There was no immediate comment from Israeli authorities.

The White House did not immediately respond to a request for comment on Haniyeh.

The news, which came less than 24 hours after Israel claimed to have killed the Hezbollah commander it said was behind a deadly strike in the Israeli-occupied Golan Heights, appears to set back chances of any imminent ceasefire agreement in Gaza.

"This assassination by the Israeli occupation of Brother Haniyeh is a grave escalation that aims to break the will of Hamas," senior Hamas official Sami Abu Zuhri told Reuters.

He said Hamas would continue the path it was following, adding: "We are confident of victory."

Haniyeh, normally based in Qatar, has been the face of the Palestinian group’s international diplomacy as the war set off by the Hamas-led attack on Israel on Oct. 7 has raged in Gaza, where three of his sons were killed in an Israeli airstrike.