Hezbollah says one of its commanders has been killed outside his home in Beirut.
Hussein al-Laqis was "assassinated" in Lebanon's capital as he returned home from work around midnight, a statement issued by the group said. It did not say how he died.
The statement accused Israel of being behind the death, claiming the Israelis had tried to kill him several times but had failed.
Hezbollah also has been fighting alongside Syrian president Bashar Assad's forces in that country's civil war, which has sparked attacks across neighbouring Lebanon.
Hezbollah described al-Laqis as one of the founding members of the group, suggesting he was a high-level commander close to the party's leadership.
Lebanese security officials said assailants opened fire on al-Laqis with an assault rifle while he was in his car, parked at the residential building in the Hadath neighbourhood.
He was taken to a nearby hospital but died from his wounds, the officials said.
The Lebanese state news agency later published a photograph it identified as al-Laqis. The image showed a man who appeared to be in his mid-40s, with neatly cut black hair and a graying close-cropped beard, wearing beige-and-khaki military clothing.
Hezbollah said: "The Israeli enemy is naturally directly to blame. This enemy must shoulder complete responsibility and repercussions for this ugly crime and its repeated targeting of leaders and cadres of the resistance."
Israeli foreign ministry spokesman Yigal Palmor denied Israeli involvement.
"Israel has nothing to do with this incident. These automatic accusations are an innate reflex with Hezbollah. They don't need evidence, they don't need facts, they just blame anything on Israel."
Hezbollah has fought several wars against Israel. Al-Laqis' son died fighting Israel in the month-long 2006 war. Israel's spy service has been suspected of assassinating Hezbollah commanders for more than two decades.
In 1992, Israeli helicopter gunships ambushed the motorcade of Hezbollah leader Sheik Abbas Musawi, killing him, his wife, five-year-old son and four bodyguards. Eight years earlier, Hezbollah leader Sheik Ragheb Harb was gunned down in south Lebanon.
One of the biggest blows for the group came in 2005 when Imad Mughniyeh, a top Hezbollah military commander, was killed by a bomb in his car in Damascus.
Current Hezbollah leader Sheik Hassan Nasrallah has rarely appeared in public since the 2006 war. In a very rare move, he travelled to neighbouring Syria last week to meet the Syrian and Iranian presidents.