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An Israeli strike on southern Lebanon on Monday killed a senior commander in Hezbollah's elite Radwan force.
Wissam Tawil, commonly known as Hajj Jawad, is the highest-level commander to be killed since cross-border clashes between Israel and Hezbollah began.
Hezbollah said in a statement that it mourned Mr Tawil "with great pride and honour".
A security source told Reuters the killing was a "very painful strike". Another security source warned that "things will flare up now".
But a Hezbollah source told The National that while he was "the first commander of his kind to be killed since the war in the south began", his death did not represent a major military loss for the group.
"The resistance is not a list of individuals that can be killed one by one. It’s a comprehensive movement," he said.
Mr Tawil was an adviser to Hezbollah on matters of "military strategy" in southern Lebanon, the source added.
Mr Tawil was from the southern Lebanese town of Khirbet Salam, where he was killed. The Hezbollah source said Mr Tawil's car was struck as he was driving through the town. No one was in the vehicle with him, the source added.
More than 150 Hezbollah fighters have been killed in Israeli strikes since cross-border clashes began after Hamas's attack on October 7.
Hezbollah was quick to act in support of its ally Hamas, which controls the Gaza Strip, by opening a front against Israel. Since October 8, Hezbollah and its allies have waged a cross-border conflict against Israel to deter its military from its assault on Gaza.
Other Iran-backed groups, including Yemen's Houthi rebels and Iraq's Kataib Hezbollah, have added to those efforts to force Israel out of Gaza.
The fighting along the Lebanon-Israel border has displaced tens of thousands, with neither Israeli or Lebanese able to return to their hometowns.
Hezbollah has repeatedly said it will continue operations along the border until a permanent ceasefire is declared. It has mostly launched its attacks against military installations and appears to be wary of the clashes developing into a full-fledged war.
Israel has intensified its strikes on Lebanon in an attempt to push Hezbollah away from the border, hitting deeper into Lebanese territory with greater regularity.
Last week, Hamas deputy Saleh Al Arouri was assassinated by what is thought to be an Israeli drone strike in Dahieh, a suburb of Beirut. Dahieh is also considered to be Hezbollah's seat of power and is where Mr Al Arouri was living.
Hezbollah leader Hassan Nasrallah previously warned that assassinations by Israel in Lebanon would “not be tolerated".
"We will not allow Lebanon to become a new field of assassinations for Israel,” he said.
Mr Tawil's killing comes as US Secretary of State Antony Blinken meets regional leaders on a tour of the Middle East. Mr Blinken has spoken of the need to contain the conflict in Gaza and avoid it from escalating into a wider regional war.
